Biochimica et Biophysica Acta (BBA) - Reviews on Biomembranes最新文献

SH2/SH3 domain-containing adaptor proteins play a critical role in regulating tyrosine kinase signalling pathways. The major function of these adaptors, such as Grb2, Nck, and Crk, is to recruit proline-rich effector molecules to tyrosine-phosphorylated kinases or their substrates. In recent years dozens of novel proteins have emerged that are capable of associating with the SH2 and the SH3 domains of adaptors. In this review, the author attempts to summarise these novel binding partners of Grb2, Nck, and Crk, and to discuss current controversies regarding function and regulation of protein multicomplexes held together by SH2/SH3 adaptor molecules at the plasma membrane.

As a result of recent genome sequencing projects as well as detailed biochemical, molecular genetic and physiological experimentation on representative transport proteins, we have come to realize that all organisms possess an extensive but limited array of transport protein types that allow the uptake of nutrients and excretion of toxic substances. These proteins fall into phylogenetic families that presumably reflect their evolutionary histories. Some of these families are restricted to a single phylogenetic group of organisms and may have arisen recently in evolutionary time while others are found ubiquitously and may be ancient. In this study we conduct systematic phylogenetic analyses of 26 families of transport systems that either had not been characterized previously or were in need of updating. Among the families analyzed are some that are bacterial-specific, others that are eukaryotic-specific, and others that are ubiquitous. They can function by either a channel-type or a carrier-type mechanism, and in the latter case, they are frequently energized by coupling solute transport to the flux of an ion down its electrochemical gradient. We tabulate the currently sequenced members of the 26 families analyzed, describe the properties of these families, and present partial multiple alignments, signature sequences and phylogenetic trees for them all.

Implications of the data on lipid–protein interactions involving integral proteins that are obtained from EPR spectroscopy with spin-labelled lipids in membranes are reviewed. The lipid stoichiometry, selectivity and exchange dynamics at the lipid–protein interface can be determined, in addition to information on the configuration and rotational dynamics of the protein-associated lipid chains. These parameters, particularly the stoichiometry and selectivity, are directly related to the intramembranous structure and degree of oligomerisation of the integral protein, and conversely may be used to study the state of assembly of such proteins in the membrane. Insertion of proteins into membranes can be studied by analogous methods. Comparison with the results obtained from integral proteins helps to define the extent of membrane penetration and degree of transmembrane crossing that are relevant to protein translocation mechanisms.

Ca²⁺-ATPase can be purified from the skeletal muscle of sarcoplasmic reticulum and reconstituted into phospholipid bilayers of defined composition. This allows a detailed study of the interactions between phospholipid molecules and the ATPase, and of the effects of phospholipid structure on the activity of the ATPase. It has been shown that the thickness of the lipid bilayer, its physical phase and the lipid headgroup structure can all be important. The interaction between phospholipids and the ATPase is not structurally specific in that the strength of the phospholipid-ATPase interaction does not depend on headgroup structure or on fatty acyl chain length, but the strength of binding is different for liquid crystalline and gel phase lipid. There are also ‘specific’ sites for some lipids on the ATPase. There is no unique mechanism explaining the effects of phospholipid on the function of the ATPase; the changes observed with any particular phospholipid follow from a distinct set of changes in the conformational state of the ATPase. The changes in activity are likely to follow from tilting of trans-membrane α-helices in the ATPase. In simple model systems it has been shown that the extent to which lipids can distort to match the protein is limited.

Phospholipids play essential roles in defining the membrane permeability barrier, in regulating cellular processes, in providing a support for organization of many membrane-associated processes, and in providing precursors for the synthesis of macromolecules. Although in vitro experiments have provided important information on the role of protein–lipid interactions in cell function, such approaches are limited by the lack of a direct measure for phospholipid function. Genetic approaches can provide direct evidence for a specific role for phospholipids in cell function provided cell viability or membrane structure is not compromised. This review will summarize recent genetic approaches that when coupled with biochemical studies have led to a better understanding of specific functions for phospholipids at the molecular level.

The formation of sphingolipid-cholesterol microdomains in cellular membranes has been proposed to function in sorting and transport of lipids and proteins as well as in signal transduction. An increasing number of cell biological and biochemical studies now supports this concept. Here we discuss the structural properties of lipids in a cell biological context. The sphingolipid-cholesterol microdomains or rafts are described as dispersed liquid ordered phase domains. These domains are dynamic assemblies to which specific proteins are selectively sequestered while others are excluded. The proteins associated to rafts can act as organizers and can modulate raft size and function.

The classical human pancreatic lipase (HPL), the guinea pig pancreatic lipase-related protein 2 (GPLRP2) and the phospholipase A1 from hornet venom (DolmI PLA1) illustrate three interesting steps in the molecular evolution of the pancreatic lipase gene family towards different substrate selectivities. Based on the known 3D structures of HPL and a GPLRP2 chimera, as well as the modeling of DolmI PLA1, we review here the structural features and the kinetic properties of these three enzymes for a better understanding of their structure–function relationships. HPL displays significant activity only on triglycerides, whereas GPLRP2 displays high phospholipase and galactolipase activities, together with a comparable lipase activity. GPLRP2 shows high structural homology with HPL with the exception of the lid domain which is made of five amino acid residues (mini-lid) instead of 23 in HPL. The lid domain deletion in GPLRP2 allows the free access to the active site and reduces the steric hindrance towards large substrates, such as galactolipids. The role of the lid domain in substrate selectivity has been investigated by site-directed mutagenesis and the substitution of HPL and GPLRP2 lid domains. The addition of a large-size lid domain in GPLRP2 increases the substrate selectivity for triglycerides by depressing the phospholipase activity. The phospholipase activity is, however, not induced in the case of the HPL mutant with GPLRP2 mini-lid. Therefore, the presence of a full-length lid domain is not the unique structural feature explaining the absence of phospholipase activity in HPL. The 3D structure of the GPLRP2 chimera and the model of DolmI PLA1 reveal a higher hydrophilic/lipophilic balance (HLB) of the surface loops (β5 loop, β9 loop, lid domain) surrounding the active site, as compared to the homologous loops in HPL. This observation provides a potential explanation for the ability of GPLRP2 and DolmI PLA1 to hydrolyze polar lipids, such as phospholipids. In conclusion, the β5 loop, the β9 loop, and the lid domain play an essential role in substrate selectivity towards triglycerides, phospholipids and galactolipids.

It has long been appreciated that lipids, particularly anionic phospholipids, promote blood coagulation. The last two decades have seen an increasing insight into the kinetic and mechanistic aspects regarding the mode of action of phospholipids in blood coagulation. This essay attempts to review these developments with particular emphasis on the structure of lipid-binding domains of blood coagulation proteins, and the variable effect of phospholipid composition on the interaction with these proteins. Some examples are discussed of how lipid membranes direct the pathway of enzymatic conversions in blood coagulation complexes, also illustrating that the membrane lipid surface is more than an inert platform for the assembly of coagulation factors. Finally, the controlled exposure of procoagulant lipid on the surface of blood cells is shortly reviewed, and an example is discussed of how interference with lipid–protein interactions in blood coagulation may result in pathological phenomena.