ISOLATION AND CHARACTERIZATION OF A CDNA-ENCODING A XENOPUS 70-KDA HEAT-SHOCK COGNATE PROTEIN, HSC70.I

We isolated a full-length cDNA clone encoding a Xenopus laevis 70-kDa heat shock cognate protein, hsc70.I. The protein coding region exhibit s a high degree of identity with a number of mammalian hsc70 proteins, such as rat hsc71 (92%), whereas the identity to Xenopus hsp70 is onl y 80%. These data suggest that the inducible and constitutive forms of hsp70 diverged long before the emergence of amphibians. The Xenopus h sc70.I contains a number of conserved elements, including the ATP-bind ing domain, a nuclear localization signal and the carboxy-terminal EEV D motif, which has been implicated in several activities associated wi th chaperonin function. Northern blot analyses revealed that maternal hsc70.I mRNA is present in cleavage and early blastula stages of Xenop us development. After the onset of zygotic transcription at the midbla stula stage, the levels of hsc70.I message increase through to the tad pole stages. Furthermore, in contrast to hsp70 mRNA, the relative leve ls of hsc70.I mRNA are not enhanced after heat shock in embryos and in the kidney epithelial cell line, A6. The levels of hsc70.I mRNA are h igh in adult spleen and testis, with moderate levels in eye, heart, li ver and brain and comparatively low levels in hindlimb muscle.