Dynamics in the Phytophthora capsici effector AVR3a11 confirm the core WY domain fold

Oomycete pathogens cause large economic losses in agriculture through diseases such as late blight (Phytophthora infestans), and stem and root rot of soybean (Phytophthora sojae). The effector protein AVR3a, from P. infestans, and its homologue AVR3a11 from P. capsici, are examples of host-translocated effectors that interact with plant proteins to evade defence mechanisms and enable infection. Both proteins belong to the family of RXLR effectors and contain an N-terminal secretion signal, an RXLR motif for translocation into the host cell, and a C-terminal effector domain. Within this family, a large number of proteins have been predicted to contain one or more WY domains as their effector domain, and this domain is proposed to encompass a conserved minimal core fold containing three helices, further stabilised by additional helices or dimerization. In AVR3a11, a helical N-terminal extension to the core fold forms a four-helix bundle, as determined by X-ray crystallography. For a complete picture of the dynamics of AVR3a11, we have determined the solution structure of AVR3a11, and studied its dynamics in the fast timescale (ns-ps, from NMR relaxation parameters) and in the slow timescale (seconds to minutes, from hydrogen/deuterium exchange experiments). Hydrogen/deuterium exchange showed that the N-terminal helix is less stable than the other three helices, confirming the core fold originally proposed. Relaxation measurements confirm that AVR3a11 undergoes extensive conformational exchange, despite the uniform presence of fast motions in the spectral density function throughout most of its sequence. As functional residues are located in the more mobile regions, this flexibility in the slow/intermediate timescale may be functionally important.