Phosphorylation-induced Change of the Oligomerization State of αB-crystallin*

aB-crystallin in cells can be phosphorylated at three serine residues in response to stress or during mitosis (Ito, H., Okamoto, K., Nakayama, H., Isobe, T., and Kato, K. (1997) J. Biol. Chem. 272, 29934–29941 and Kato, K., Ito, H., Kamei, K., Inaguma, Y., Iwamoto, I., and Saga, S. (1998) J. Biol. Chem. 273, 28346–28354). In the present study, we determined effects of phosphorylation of aBcrystallin 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-aB). From results of sucrose density gradient centrifugation, we found that wild type aB-crystallin (wt-aB) and 3D-aB sedimented in fractions corresponding to apparent molecular masses of about 500 and 300 kDa, respectively. Chaperone-like activity of 3D-aB was significantly weaker than that of wt-aB. When wt-aB and 3D-aB 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, aBcrystallin 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-aB or 3D-aB in Chinese hamster ovary cells caused protection of the enzyme from heat inactivation although the degree of protection with 3D-aB was less than that with wt-aB. From these observations, it is suggested that phosphorylation of aB-crystallin causes dissociation of large oligomers to smaller sizes molecules and reduction of chaperone-like activity, like in the case of HSP27.