THE 13-KD FK506 BINDING-PROTEIN, FKBP13, INTERACTS WITH A NOVEL HOMOLOG OF THE ERYTHROCYTE-MEMBRANE CYTOSKELETAL PROTEIN-4.1

We have identified a novel generally expressed homologue of the erythr ocyte membrane cytoskeletal protein 4.1, named 4.1G, based on the inte raction of its COOH-terminal domain (CTD) with the immunophilin FKBP13 , The 129-amino acid peptide, designated 4.1G-CTD, is the first known physiologic binding target of FKBP13. FKBP13 is a 13-kD protein origin ally identified by its high affinity binding to the immunosuppressant drugs FK506 and rapamycin (Jin, Y., M.W. Albers, W.S. Lane, B.E. Biere r, and S.J. Burakoff, 1991. Proc. Natl. Acad. Sci, USA. 85:6677-6681); it is a membrane-associated protein thought to function as an ER chap erone (Bush, K.T., B.A. Henrickson, and S.K. Nigam. 1994. Biochem. J. [Tokyo]. 303:705-708). We report the specific association of FKBP13 wi th 4.1G-CTD based on yeast two-hybrid, in vitro binding and coimmunopr ecipitation experiments. The histidyl-proline moiety of 4.1G-CTD is re quired for FKBP13 binding, as indicated by yeast experiments with trun cated and mutated 4.1G-CTD constructs. In situ hybridization studies r eveal cellular colocalizations for FKBP13 and 4.1G-CTD throughout the body during development, supporting a physiologic role for the interac tion. Interestingly, FKBP13 cofractionates with the red blood cell hom ologue of 4.1 (4.1R) in ghosts, inside-out vesicles, and Triton shell preparations. The identification of FKBP13 in erythrocytes. which lack ER, suggests that FKBP13 may additionally function as a component of membrane cytoskeletal scaffolds.