The process of mRNA splicing is sensitive to in
vivo thermal inactivation, but can be protected by pretreatment
of cells under conditions that induce heat-shock proteins
(Hsps). This latter phenomenon is known as “splicing
thermotolerance”. In this article we demonstrate
that the small nuclear ribonucleoprotein
particles (snRNPs) are in vivo targets of thermal
damage within the splicing apparatus in heat-shocked yeast
cells. Following a heat shock, levels of the tri-snRNP
(U4/U6.U5), free U6 snRNP, and a pre-U6 snRNP complex are
dramatically reduced. In addition, we observe multiple
alterations in U1, U2, U5, and U4/U6 snRNP profiles and
the accumulation of precursor forms of U4- and U6-containing
snRNPs. Reassembly of snRNPs following a heat shock is
correlated with the recovery of mRNA splicing and requires
both Hsp104 and the Ssa Hsp70 family of proteins. Furthermore,
we correlate splicing thermotolerance with the protection
of a subset of snRNPs by Ssa proteins but not Hsp104, and
show that Hsp70 directly associates with U4- and U6-containing
snRNPs in splicing thermotolerant cells. In addition, our
results show that Hsp70 plays a role in snRNP assembly
under normal physiological conditions.