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 co nstitutively expressed during oogenesis was investigated during embryo genesis of the amphibian Pleurodeles waltl. Results from Northern hybr idization experiments and RNase protection assays provided evidence fo r the presence of inducible hsp70 mRNA under normal conditions at ever y embryonic stage. Immunoblotting of embryo proteins separated by 2D-e lectrophoresis provided evidence for the presence of a single polypept ide of about 74 kDa likely to be an HSP70-related protein, from unfert ilized egg to tailbud stage. Immunocytological analysis showed that HS P70-related proteins were localized in the cytoplasm of all blastomere s. It also pointed out that nuclear transfer of the protein occurs in certain cells, precisely at the time of their invagination and subsequ ent internalization during normal Pleurodeles development. Such nuclea r transfer involves involuting mesodermal cells in the blastopore regi on at the time of gastrulation. It also involves neurodermic cells at the time of neural tube closure. Interestingly, in exogastrulas nuclea r transfer did not occur in cells which could no longer invaginate. Su ch 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.