The phosphorylation of stathmin by MAP kinase

Stathmin is a major substrate for mitogen-activated protein kinase during heat shock and chemical stress in HeLa cells

Stathmin is a ubiquitous, highly conserved 19-kDa cytoplasmic protein whose expression and phosphorylation are regulated in relation to cell proliferation, differentiation or activation, in many biological systems. In this report, we show that stathmin undergoes major phosphorylation in HeLa cells submitted to heat or chemical stress. Heat-shock-induced stathmin phosphorylation was very rapid, as maximal incorporation of phosphate was observed at 5 min. Phosphorylation of stathmin might, therefore, occur as a very early step in the intracellular response to heat shock. The sites of phosphorylation of stathmin involved during the stress response were identified as mostly Ser25 and, to a lesser extent, Ser38. These sites are both followed by a proline residue, and known to be good substrates in vitro for mitogen-activated protein kinase (MAP-kinase) and p34cdc2 kinase, respectively. In lysates from heat-shocked cells, an increased stathmin-kinase activity, distinct from the histone-H1-kinase activity, was found to phosphorylate stathmin mostly on Ser25, the main site for MAP-kinase in vitro. This stathmin-kinase coeluted quantitatively with the stress-activated MAP-kinase from an FPLC MonoQ column. Furthermore, a stathmin kinase activity was precipitated from lysates of heat-shocked HeLa cells by an anti-(MAP-kinase) serum. Together, these results indicate that the phosphorylation of stathmin by MAP-kinase is likely to be a significant component of the signalling array controlling the cellular response to stress, and they further underline the general involvement of stathmin in intracellular signalling.

Stathmin gene expression in mammary gland and in Nb2 cells

Mammary gland growth occurs essentially during pregnancy and induction of milk synthesis is triggered at parturition. Prolactin is mammogenic in vivo but only marginally in vitro. Prolactin induces milk synthesis in vivo and in cultured mammary cells. Prolactin is also strictly required for the multiplication of the rat lymphoid Nb2 cells. Stathmin is an ubiquitous and highly conserved phosphoprotein which seems to be involved in the intracellular mechanisms which trigger cell multiplication and differentiation. In the present study, the concentration of stathmin mRNA has been evaluated during the pregnancy-lactation-weaning cycle in mouse and rabbit. Stathmin mRNA appeared at its highest level during pregnancy and it was almost undetectable during lactation. Prolactin injected into mid-pregnant rabbits induced milk synthesis and this effect was not accompanied by any modification of stathmin mRNA concentration. In cultured primary rabbit mammary cells, prolactin induced casein gene expression without any alteration of stathmin mRNA concentration. In Nb2 cells, prolactin induced a progressive increase of stathmin mRNA concentration. This effect was not significant until after 4 h of prolactin action. These data suggest that stathmin is involved in mammary and Nb2 cell multiplication but may not be necessary for mammary cell differentiation.

Molecular characterization of human stathmin expressed in Escherichia coli: site-directed mutagenesis of two phosphorylatable serines (Ser-25 and Ser-63)

Stathmin, a probable relay protein possibly integrating multiple intracellular regulatory signals [reviewed in Sobel (1991) Trends Biochem. Sci. 16, 301-305], was expressed in Escherichia coli at levels as high as 20% of total bacterial protein. Characterization of the purified recombinant protein revealed that it had biochemical properties very similar to those of the native protein. It is a good substrate for both cyclic AMP-dependent protein kinase (PKA) and p34cdc2, on the same four sites as the native eukaryotic protein. As shown by m.s., the difference in isoelectric points from the native protein is probably due to the absence of acetylation of the protein produced in bacteria. C.d. studies indicate that stathmin probably contains about 45% of its sequence in an alpha-helical conformation, as also predicted for the sequence between residues 47 and 124 by computer analysis. Replacement of Ser-63 by alanine by in vitro mutagenesis resulted in a ten times less efficient phosphorylation of stathmin by PKA which occurred solely on Ser-16, confirming that Ser-63 is the major target of this kinase. Replacement of Ser-25, the major site phosphorylated by mitogen-activated protein kinase in vitro and in vivo, by the charged amino acid glutamic acid reproduced, in conjunction with the phosphorylation of Ser-16 by PKA, the mobility shift on SDS/polyacrylamide gels induced by the phosphorylation of Ser-25. This result strongly suggests that glutamic acid in position 25 is able to mimic the putative interactions of phosphoserine-25 with phosphoserine-16, as well as the resulting conformational changes that are probably also related to the functional regulation of stathmin.

The threonine residues in MAP kinase kinase 1 phosphorylated by MAP kinase in vitro are also phosphorylated in nerve growth factor-stimulated rat phaeochromocytoma (PC12) cells

The residues on MAP kinase kinase-1 (MAPKK1) phosphorylated by MAP kinase in vitro have been identified as Thr-291 and Thr-385. Both threonines are phosphorylated in PC12 cells and the 32P-labelling of each residue increases after stimulation with nerve growth factor (NGF). The results establish that MAPKK1 is a physiological substrate for MAP kinase. The two active forms of MAPKK that are resolved by Mono Q chromatography of PC12 cell extracts are both phosphorylated at Thr-291 and Thr-385, demonstrating that neither species is the MAPKK2 isoform which lacks Thr-291.

Deciphering the MAP kinase pathway

MAP kinases (MAPK) are serine/threonine kinases which are activated by a dual phosphorylation on threonine and tyrosine residues. Their specific upstream activators, called MAP kinase kinases (MAPKK), constitute a new family of dual-specific threonine/tyrosine kinases, which in turn are activated by upstream MAP kinase kinase kinases (MAPKKK). These three kinase families are successively stimulated in a cascade of activation described in various species such as mammals, frog, fly, worm or yeast. In mammals, the MAP kinase module lies on the signaling pathway triggered by numerous agonists such as growth factors, hormones, lymphokines, tumor promoters, stress factors, etc. Targets of MAP kinase have been characterized in all subcellular compartments. In yeast, genetic epistasis helped to characterize the presence of several MAP kinase modules in the same system. By complementation tests, the relationships existing between phylogenetically distant members of each kinase family have been described. The roles of the MAP kinase cascade have been analyzed by engineering various mutations in the kinases of the module. The MAP kinase cascade has thus been implicated in higher eukaryotes in cell growth, cell fate and differentiation, and in low eukaryotes, in conjugation, osmotic stress, cell wall construct and mitosis.