Phosphorylation-induced Change of the Oligomerization State of αB-crystallin
Journal of Biological Chemistry
␣B-crystallin in cells can be phosphorylated at three serine residues in response to stress or during mitosis (present study, we determined effects of phosphorylation of ␣Bcrystallin on its oligomerization state, mainly by using site-directed mutagenesis, in which all three phosphorylation sites were substituted with aspartate to mimic the phosphorylation state (3D-␣B). From results of sucrose density gradient centrifugation, we found that wild type ␣B-crystallin (wt-␣B) and 3D-␣B sedimented in
... 3D-␣B sedimented in fractions corresponding to apparent molecular masses of about 500 and 300 kDa, respectively. Chaperone-like activity of 3D-␣B was significantly weaker than that of wt-␣B. When wt-␣B and 3D-␣B were expressed in COS-m6 cells, they sedimented at positions corresponding to apparent molecular masses of about 500 and 100 kDa, respectively. In U373 MG human glioma cells, ␣Bcrystallin was observed as large oligomers with apparent molecular masses about 500 kDa and the oligomerization size was reduced after phosphorylation induced by phorbol 12-myristate 13-acetate and okadaic acid. Coexpression of luciferase and wt-␣B or 3D-␣B in Chinese hamster ovary cells caused protection of the enzyme from heat inactivation although the degree of protection with 3D-␣B was less than that with wt-␣B. From these observations, it is suggested that phosphorylation of ␣B-crystallin causes dissociation of large oligomers to smaller sizes molecules and reduction of chaperone-like activity, like in the case of HSP27.