C-TERMINAL TRIMERIZATION, BUT NOT N-TERMINAL TRIMERIZATION, OF THE REOVIRUS CELL ATTACHMENT PROTEIN IS A POSTTRANSLATIONAL AND HSP70 ATP-DEPENDENT PROCESS/

The C-terminal globular head of the lollipop-shaped sigma 1 protein of reovirus is responsible for interaction with the host cell receptor. Like the N-terminal fibrous tail, it has its own trimerization domain, Whereas N-terminal trimerization (formation of a triple alpha-helical coiled coil) occurs at the level of polysomes (i.e. cotranslationally ) and is ATP-independent, C-terminal trimerization is a posttranslatio nal event that requires ATP. Coprecipitation experiments using anti-Hs p70 antibodies and truncated sigma 1 proteins synthesized in vitro rev ealed that only regions downstream of the N-terminal alpha-helical coi led coil were associated with Hsp70. Hsp70 was also found to be associ ated with nascent al chains on polysomes as well as with immature post ribosomal sigma 1 trimers (hydralike intermediates with assembled N te rmini and unassembled C termini). These latter structures were true in termediates in the al biogenetic pathway since they could be chased in to mature al trimers with the release of Hsp70. Thus, unlike N-termina l trimerization, C-terminal trimerization is Hsp70- and ATP-dependent. The involvement of two mechanistically distinct oligomerization event s for the same molecule, one cotranslational and one posttranslational , may represent a common approach to the generation of oligomeric prot eins in the cytosol.