ONLY THE FIRST AND THE LAST HYDROPHOBIC SEGMENTS IN THE COOH-TERMINAL3RD OF NA,K-ATPASE ALPHA-SUBUNIT INITIATE AND HALT, RESPECTIVELY, MEMBRANE TRANSLOCATION OF THE NEWLY SYNTHESIZED POLYPEPTIDE - IMPLICATIONS FOR THE MEMBRANE TOPOLOGY
Yh. Xie et al., ONLY THE FIRST AND THE LAST HYDROPHOBIC SEGMENTS IN THE COOH-TERMINAL3RD OF NA,K-ATPASE ALPHA-SUBUNIT INITIATE AND HALT, RESPECTIVELY, MEMBRANE TRANSLOCATION OF THE NEWLY SYNTHESIZED POLYPEPTIDE - IMPLICATIONS FOR THE MEMBRANE TOPOLOGY, The Journal of biological chemistry, 271(5), 1996, pp. 2563-2573
We studied the topogenic properties of five hydrophobic segments (H5-H
9) in the COOH-terminal third of Na,K-ATPase alpha subunit using in vi
tro insertion of fusion proteins into endoplasmic reticulum membranes,
These fusion proteins consisted of several different lengths of trunc
ated alpha subunit starting at Met(729) and a reporter protein, chlora
mphenicol acetyltransferase, that was linked in frame after each hydro
phobic segment. We found that membrane insertion of the newly synthesi
zed COOH-terminal third was initiated by H5 and terminated by H9, indi
cating that here only H5 and H9 have topogenic function. The other thr
ee, H6-H8, did not have topogenic function in the native context and w
ere translocated into the endoplasmic reticulum lumen. These results w
ere in striking contrast to the previous models in which four or six h
ydrophobic segments were proposed to cross the membrane. Furthermore,
the findings suggest a novel mechanism for achieving the final membran
e topology of the COOH-terminal third of the alpha subunit.