IN-VITRO FUNCTIONAL-CHARACTERIZATION OF BACTERIALLY EXPRESSED HUMAN FIBROBLAST TROPOMYOSIN ISOFORMS AND THEIR CHIMERIC MUTANTS

At least eight tropomyosin isoforms (hTM1, hTM2, hTM3, hTM4, hTM5, hTM 5a, hTM5b, and hTMsmalpha) are expressed from four distinct genes in h uman fibroblasts. In order to elucidate isoform properties, we have su bcloned hTM3 and hTM5 full-length cDNAs, as well as their chimeric cDN As into the bacterial expression pET8C system. Bacterially expressed t ropomyosin isoforms (called PEThTM3, PEThTM5, PEThTM5/3, and PEThTM3/5 ) were purified and characterized. Under optimal binding conditions, t he binding of PEThTM5 isoform to F-actin was stronger than the PEThTM3 isoform. However, analysis of actin-binding by the McGhee and von Hip pel equation revealed that PEThTM3 exhibits higher cooperativity in bi nding than PEThTM5 does. Furthermore, the chimera PEThTM5/3 which poss essed the N-terminal fragment of hTM5 fused to the C-terminal fragment of hTM3 had even stronger actin binding ability. The reverse chimera PEThTM3/5 which possessed the N-terminal fragment of hTM3 fused to the C-terminal fragment of hTM5 demonstrated greatly reduced affinity to actin filaments. In addition, both chimeras had different KCl requirem ents for optimal binding to F-actin than their parental tropomyosins. A bacterially made C-terminal fragment of human fibroblast caldesmon ( PETCaD39) and native chicken gizzard caldesmon were both able to enhan ce the actin-binding of these bacterially expressed tropomyosins. Howe ver, PETCaD39's enhancement of binding to F-actin was greater for PETh TM5 than PEThTM3. Under 30 mM KCl and 4 mM MgCl2, the low M(r) isoform PEThTM5 appeared to be able to amplify the actin-activated HMM ATPase activity by 4.7 fold, while the high M(r) isoform PEThTM3 stimulated the activity only 1.5 fold. The higher enhancement of ATPase activity by PEThTM5 than by PEThTM3 suggested that the low M(r) isoform hTM5 ma y be more involved in modulating nonmuscle cell motility than hTM3. Th ese results further suggested that different isoforms of tropomyosin m ight have finite differences in their specific functions (e.g., cytosk eletal vs. motile) inside the cell. (C) 1993 Wiley-Liss, Inc.