MAP klna~e.acttvatcd protein klna,e-= (MAPKAP klntts~.) phosphorylate, the sed,e t~.stdu~ in marine heat 1hock ~ ~(hlp~)m,lhuaumshock prot©ln is present in rabbit skeletal mu,¢le and hsl~$ kinue activity Ill skeletal mu~le extracts co~urtfln with MAPKAP klaaze~ actht~ throughout the purllicatton or the hitter enzyme. These p~sultm SUl;Sest that MAPKAP k|lla,t.-2 is the en~ me zeq~ondble f~r the photphoqlMim o1' these small heat shock protelm in mammalian ~elh.MAP klnase; Protein ktnase~ Heal shock~ Protein phosphorylatlon: Oro~lh factor i, INTRODUCTIONWe have recently identified a new protein kinase in rabbit skeletal muscle which is only active after it has been phosphorylatcd on a unique threonine residue by mitogen-activated protein kinase (MAP kinase) [1]. This enzyme, which has been termed MAP kinase activated ]2rotein kinase-2 (MAPKAP kinase-2) can be-distinguished from $6 kinase-II (or MAPKAP kinase-l) [2], the only other protein kinase known to be activated by MAP kinase, by its response to inhibitors, failure to phosphorylate peptides related to the C-temlinus of ribosomal protein $6 and by its amino acid sequence [1]. MAPKAP kinase-2 was originally identified by its ability to phosphorylat¢ rabbit skeletal muscle gl~.'ogen synthase, which it labels preferentially on a serine located seven residues from the N-terminus. It also phosphorylates the first serine in the peptide KKPLNRTLSVASLPGLamide, which is related to the N-terminus of glycogen synthase, and this substrate is used to assay MAPKAP kinase-2 routinely [I], Although glycogen synthase was the first substrate for MAPKAP kinase-2 to be identified, it is not clear whether it is phosphorylated by this protein kinase in vice, Furthermore, since MAPKAP kinase-i may phosphorylate more than one substrate in vice (e.g. the glycogen-bindin 8 subunit of protein phosphatase- ribosomal protein $6 [4]), MAPKAP kinme-2 may aim have a number of physiological substmtes and thereby mediate everal actions of extraodlular dSuab which exert their effects through the activation of MAP idmu~ and its downstream targets. We were lh¢~occ interested in identifying potential ph)~olosical subslrates for MAPKAP kinase-2.Murine heat shock protein ~ (hsp25) and its human homologue heat shock protein 27 (hsp27) are small thermostable proteins present in almost all nmmmalian cells, which become phosphorylated in many cells in response to signals such as tumour necrosis factor (TNF) [5,6], intedeuk,.'n-I [6,7], platelet derived growth factor (PDGF) [7] and fibroblast growth factor (FGF) [7,8], as well as turnout-promoting phorboi esters [9,10| and •heat shock [1 I--13], Their physiological roles are unknown, although overexpression of hsp27 and hsp25 has been reported to increase the thermotoleranceof some mammalian cells [14.15] and to inhibit cell proliferation [15|. Our interest in examining whether hsp25 and hsp27 x~re ph3-~ioiogical substrates for MAPKAP kinase-2 was amused by two observations. Firstly, several of the stimuli which trigger the phosphorylation of hsp25 a...