We have cloned the entire coding region of a mouse germ cell-specific cDNA encoding a unique protein kinase whose catalytic domain contains only three consensus subdomains (I-III) instead of the normal 12. The protein possesses intrinsic Ser/Thr kinase activity and is exclusively expressed in haploid germ cells, localizing only in their nuclei, and was thus named Haspin (for haploid germ cell-specific nuclear protein kinase). Western blot analysis showed that specific antibodies recognized a protein of M r 83,000 in the testis. Ectopically expressed Haspin was detected exclusively in the nuclei of cultured somatic cells. Even in the absence of kinase activity, however, Haspin caused cell cycle arrest at G 1 , resulting in growth arrest of the transfected somatic cells. In a DNA binding experiment, approximately onehalf of wild-type Haspin was able to bind to a DNAcellulose column, whereas the other half was not. In contrast, all of the deletion mutant Haspin that lacked autophosphorylation bound to the DNA column. Thus, the DNA-binding activity of Haspin may, in some way, be associated with its kinase activity. These observations suggest that Haspin has some critical roles in cell cycle cessation and differentiation of haploid germ cells.
CD151, one of the tetraspanins, forms a stable complex with integrin ␣31, the major laminin receptor on the cell surface. We found that 8C3, an anti-CD151 mAb obtained by screening for reactivity with integrin ␣31-CD151 complexes, was capable of dissociating CD151 from integrin ␣31, thereby allowing us to deplete CD151 from purified integrin ␣31-CD151 complexes. The CD151-free integrin ␣31 thus obtained showed a significant reduction in its ability to bind to laminin-10͞11, a high-affinity ligand for integrin ␣31, with a concomitant reduction in its reactivity with mAb AG89, which recognizes activated 1-containing integrins. These results raised the possibility that the association of integrin ␣31 with CD151 potentiates the ligand-binding activity of the integrin through sustaining its activated conformation. In support of this possibility, the ligand-binding activity was restored when CD151-free integrin ␣31 was reassociated with purified CD151. 8C3-induced dissociation of CD151 from integrin ␣31 was also demonstrated on the surface of living cells by fluorescent resonance energy transfer imaging, accompanied by a concomitant reduction in the cell adhesion to laminin-10͞11-coated substrates. CD151 knock-down by RNA interference also resulted in a reduction in the adhesive activity of the cells. Taken together, these results indicate that CD151 association modulates the ligandbinding activity of integrin ␣31 through stabilizing its activated conformation not only with purified proteins but also in a physiological context. cell adhesion ͉ laminin
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