Molecular chaperones ensure that their substrate proteins reach the functional native state, and prevent their aggregation. Recently, an additional function was proposed for molecular chaperones: they serve as buffers (_capacitors_) for evolution by permitting their substrate proteins to mutate and at the same time still allowing them to fold productively.

Using pairwise alignments of _E. coli_ genes with genes from other gamma-proteobacteria, we showed that the described buffering effect cannot be observed among substrate proteins of GroEL, an essential chaperone in _E. coli_. Instead, we find that GroEL substrate proteins evolve less than other soluble _E. coli_ proteins. We analyzed several specific structural and biophysical properties of proteins to assess their influence on protein evolution and to find out why specifically GroEL substrates do not show the expected higher divergence from their orthologs.

Our results culminate in four main findings: *1.* We find little evidence that GroEL in _E. coli_ acts as a capacitor for evolution _in vivo_. *2.* GroEL substrates evolved less than other _E. coli_ proteins. *3.* Predominantly structural features appear to be a strong determinant of evolutionary rate. *4.* Besides size, hydrophobicity is a criterion for exclusion for a protein as a chaperonin substrate
