Proteins interacting, sHsps

Direct interaction of sHsps other than a-crystallin with membranes has been reported only for Synechocystis Hsp 16.6 (Torok et al., 2001). Purified recombinant protein interacted with lipid vesicles, as monitored by fluorescence anisotropy, to reduce lipid fluidity and also increased the surface pressure of lipid monolayers. There was evidence for both lipid specificity and membrane penetration of the sHsp. Thylakoids isolated from cells deleted for Hsp 16.6 were more fluid membranes than wild-type cells, even in the presence of very little sHsp. Ability to stabilize directly photosynthetic membranes by lipid interaction could explain data supporting a role for sHsps in photosystem II protection, but the specificity of this effect has not been very well documented (Harndahl and Sundby, 2001 Heckathorn et al., 1998 Lee et al., 2000 Nakamoto et al., 2000). 

The chaperone model for sHsp function provides a basic framework to explain the many proposed sHsp/protein interactions and potential functions. The diversity of the sHsp family, however, indicates that care must be taken in generalizing biochemical properties and activities across different family members. Nonetheless, we now have a firmer structural foundation on which to design future experiments to build a biochemical mechanism of action. 

Based on interactions with model substrates, an idea of the substrate conformation recognized by sHsps has been derived. First, a-crystallin does not interact with stable unfolded hydrophobic proteins such as a -lactalbumin and -casein (Carver et al., 1995), which readily interact with DnaKand SecB. Plant PsHsp 18.1 also failed to interact with reduced, carboxymethylated a-lactalbumin at room temperature (G. J. Lee and... 

In addition to their potential to interact with non-native proteins in a chaperone capacity, a recent hypothesis postulates that sHsps interact with membranes to reduce high-temperature—induced membrane fluidity (Torok et al., 2001 Vigh et al., 1998). Membranes are clearly an important cellular component perturbed by heat, and a need for membrane stabilization is logical. Changes in membrane composition and correlated fluidity in S. cerevisiae and Synechocystis change the set point ... 

The sHsps are a diverse family of proteins, which are related by possession of a common, defined a-crystallin structural domain. While virtually all of these proteins share the capacity to act as chaperones by binding non-native proteins in an ATP-independent manner, evolutionary arguments and biochemical data indicate that they can interact with many different proteins to influence a potentially wide range of functions in different cells and organisms. In this regard, they are similar to other chaperones which, by the ability to bind diverse substrates, act to protect and/or to regulate multiple cellular processes. 

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