HEAT-SHOCK PROTEINS IN AXOPLASM - HIGH CONSTITUTIVE LEVELS AND TRANSFER OF INDUCIBLE ISOFORMS FROM GLIA

To characterize heat-shock proteins (HSPs) of the 70-kDa family in the crayfish medial giant axon (MGA), we analyzed axoplasmic proteins sep arately from proteins of the glial sheath. Several different molecular weight isoforms of constitutive HSP 70s that were detected on immunob lots were approximately 1-3% of the total protein in the axoplasm of M GAs. To investigate inducible HSPs, MGAs were heat shocked in vitro or in vivo, then the axon was bathed in radiolabeled amino acid for 4 ho urs. After either heat-shock treatment, protein synthesis in the glial sheath was decreased compared with that of control axons, and newly s ynthesized proteins of 72 kDa, 84 kDa, and 87 kDa appeared in both the axoplasm and the sheath. Because these radiolabeled proteins were pre sent in MGAs only after heat-shock treatments, we interpreted the newl y synthesized proteins of 72 kDa, 84 kDa, and 87 kDa to be inducible H SPs. Furthermore, the 72-kDa radiolabeled band in heat-shocked axoplas m and glial sheath samples comigrated with a band possessing HSP 70 im munoreactivity. The amount of heat-induced proteins in axoplasm sample s was greater after a 2-hour heat shock than after a 1-hour heat shock . These data indicate that MGA axoplasm contains relatively high level s of constitutive HSP 70s and that, after heat shock, MGA axoplasm obt ains inducible HSPs of 72 kDa, 84 kDa, and 87 kDa from the glial sheat h. These constitutive and inducible HSPs may help MGAs maintain essent ial structures and functions following acute heat shock. (C) 1998 Wile y-Liss, Inc.