Does the chaperone heat shock protein hsp70 play a role in the control of developmental processes?

Expression of an hsp70 gene strictly inducible in somatic cells and constitutively expressed during oogenesis was investigated during embryogenesis of the amphibian Pleurodeles waltl. Results from Northern hybridization experiments and RNase protection assays provided evidence for the presence of inducible hsp70 mRNA under normal conditions at every embryonic stage. Immunoblotting of embryo proteins separated by 2D-electrophoresis provided evidence for the presence of a single polypeptide of about 74 kDa likely to be an HSP70-related protein, from unfertilized egg to tailbud stage. Immunocytological analysis showed that HSP70-related proteins were localized in the cytoplasm of all blastomeres. It also pointed out that nuclear transfer of the protein occurs in certain cells, precisely at the time of their invagination and subsequent internalization during normal Pleurodeles development. Such nuclear transfer involves involuting mesodermal cells in the blastopore region at the time of gastrulation. It also involves neurodermic cells at the time of neural tube closure. Interestingly, in exogastrulas nuclear transfer did not occur in cells which could no longer invaginate. Such behavior of HSP70-related proteins led us to suggest that they are involved in the control of nuclear activity associated with important developmental events such as cellular internalization processes. Such a role may be a direct consequence of HSP70-related protein functional properties as molecular chaperones.