Localization of constitutive heat shock proteins in developing ascidians.

Heat shock proteins (HSP) are a group of highly conserved proteins that regulate protein folding and ameliorate the effects of environmental stress. In the present study, the question of whether or not ascidian oocytes, embryos and larvae constitutively synthesize HSP was studied using HSP 60 and HSP 70 antibodies. Developmental stages obtained from Boltenia villosa, Cnemidocarpa finmarkiensis, Styela montereyensis and Corella willmeriana were examined for HSP using indirect immunocytochemistry. Myoplasm in oocytes and unfertilized eggs reacted with HSP 60 and 70 antibodies. HSP signals dramatically moved into the vegetal egg cytoplasm during ooplasmic segregation and colocalized with the myoplasm. In cleavage-stage embryos, HSP signals were partitioned with the myoplasm into muscle progenitor blastomeres and HSP signals were evident in the tail muscle cells of larvae. Immunoblots of proteins extracted from oocytes, eggs, embryos and larvae indicate that anti-HSP 60 recognizes a single band having an estimated molecular weight of 60 kDa. Egg centrifugation experiments suggest that most of the ascidian myoplasmic HSP are mitochondrial proteins. These results raise an intriguing possibility that mitochondria associated with the myoplasm perform biochemical functions that are unique to the embryonic muscle cell lineage.