FUNCTION OF TYPE-I AND TYPE-II KERATIN HEAD DOMAINS - THEIR ROLE IN DIMER, TETRAMER AND FILAMENT FORMATION

To examine the role of the keratin head region and its subdomains in f ilament assembly we constructed several deletion mutants of type I and type LI keratins and analysed their in vitro IF forming capacity. The Delta K8 (1-74) and Delta K18 (1-56) mutants formed only soluble olig omers, predominantly tetramers with their heterotypic partners. K8 mut ants that retained either the entire (Delta K8 (1-64)) or nearly the e ntire (Delta K8 (1-66)) H1 subdomain formed some short and irregular I F-like structures with K18. However, filaments never reached the norma l length and more protofilamentous material was observed. Analysis of the soluble complexes in 2 M guanidine-HCl indicated that tetramer for mation was impaired in the truncated molecules. The length of the dele tion correlated with the degree of tetramer destabilization. These res ults suggest that the head domain - specifically the H1 subdomain of t ype II keratins - plays a direct role in IF assembly. Its functions in clude a stabilization of the tetramer molecule, suggesting a role in d irecting the alignment of dimers as well as in elongation. We also ana lysed whether both head domains are required or if either type I or ty pe II head domains alone are sufficient for IF formation. Hybrid molec ules carrying their partner keratins head domains (K18 (8 head) and K8 (18 head)) were combined with their wild-type partners and tested for IF-forming ability. Both combinations formed filaments distinct from normal IF. The effect of the 'replaced' head domains was not compensat ed when both hybrid molecules were combined. Taken together, the resul ts indicate that complete removal of the head domains of either K8 or K18 arrested IF assembly at the state of soluble oligomers. Replacemen t of the head domains by head domains of the complementary partner par tly compensated for the effect. However, regular IF formation could no t take place when either the head domain was missing or it was replace d by the partner's keratin head.