Interspecific variation in thermal denaturation of proteins in the congeneric mussels Mytilus trossulus and M-galloprovincialis: Evidence from the heat shock response and protein ubiquitination

Individuals of two species of blue mussels, Mytilus trossulus (Could, 1850) and M. galloprovincialis (Lamarck, 1819), that have different latitudinal distributions, were collected from two locations on the Pacific coast of the USA where their distributions do not overlap. To determine if the congeners were differentially sensitive to thermal stress, we first held individuals of each species at 13 degrees C for 8 wk and then examined three biochemical indices of thermal damage to cellular proteins: relative levels of the stress protein hsp70, quantities of ubiquitin conjugates and the induction of stress-protein synthesis. The results provide evidence that the northern species, M. trossulus, was more thermally sensitive than the southern species, M. galloprovincialis. Relative levels of hsp70 and amounts of ubiquitin conjugates were higher in gill tissue from M. trossulus than in gill from M. galloprovincialis, which suggests that M. trossulus was more susceptible to reversible and irreversible protein damage, respectively, than M. galloprovincialis. In addition, the patterns of stress-protein expression as measured by in vitro radiolabeling experiments using isolated gill tissue, were significantly different, as follows: (1) the threshold induction temperatures for hsp70 synthesis were 23 and 25 degrees C for M. trossulus and M. galloprovincialis, respectively; (2) the overall intensity of synthesis and induction was greater in M. galloprovincialis than in M. trossulus, particularly at the higher incubation temperatures of 28 and 30 degrees C; (3) M. galloprovincialis expressed a 30 kdalton, stress protein that was not induced in the northern species, M. trossulus. Thus, after an 8 wk exposure to a common temperature, the two edulis-like mussel congeners appeared to be physiologically distinct with respect to thermal damage to proteins. Due to the energetic cost that is probably associated with environmentally-induced protein damage and maintaining pools of stress proteins, differential organismal thermotolerances and protein stabilites may contribute to setting species distribution-limits. Our data support conclusions of other workers that M. trossulus is a more cold-adapted species than M. galloprovincialis.