Hsp 15: a ribosome-associated heat shock protein

We are analyzing highly conserved heat shock genes of unknown or unclear fu nction with the aim of determining their cellular role. Hsp15 has previousl y been shown to be an abundant nucleic acid-binding protein whose synthesis is induced massively at the RNA level upon temperature upshift, We have no w identified that the in vivo target of Hsp15 action is the free 50S riboso mal subunit, Hsp15 binds with very high affinity (K-D <5 nM) to this subuni t, but only when 50S is free, not when it is part of the 70S ribosome, In a ddition, the binding of Hsp15 appears to correlate with a specific state of the mature, free 50S subunit, which contains bound nascent chain. This pro vides the first evidence for a so far unrecognized abortive event in transl ation, Hsp15 is suggested to be involved in the recycling of free 50S subun its that still carry a nascent chain. This gives Hsp15 a very different fun ctional role from all other heat shock proteins and points to a new aspect of translation.