Aj. Travis et al., A novel NH2-terminal, nonhydrophobic motif targets a male germ cell-specific hexokinase to the endoplasmic reticulum and plasma membrane, J BIOL CHEM, 274(48), 1999, pp. 34467-34475
Although three germ cell-specific transcripts of type 1 hexokinase exist in
murine male germ cells, only one form, HK1-sc, is found at the protein lev
el. This single isoform localizes to three distinct structures in mouse spe
rmatozoa: the membranes of the head, the mitochondria in the midpiece, and
the fibrous sheath in the flagellum (Travis, A. J., Foster, J. A. Rosenbaum
, N. A., Visconti, P. E., Gerton, G. L., Kopf, G. S., and Moss, S. B. (1998
) Mol. Biol, Cell 9, 263-276), The mechanism by which one protein is target
ed to multiple sites within this highly polarized cell poses important ques
tions of protein targeting. Because the study of protein targeting in germ
cells is hampered by the lack of established cell lines in culture, constru
cts containing different domains of the germ cell-specific hexokinase trans
cripts were linked to a green fluorescent protein and transfected into hexo
kinase-deficient M+R42 cells. Constructs containing a nonhydrophobic, germ
cell-specific domain, present at the amino terminus of the HK1-SC protein,
were targeted to the endoplasmic reticulum and the plasma membrane. Mutatio
nal analysis of this domain demonstrated that a complex motif, PKIRP-PLTE (
with essential residues italicized), represented a novel endoplasmic reticu
lum-targeting motif, Constructs based on another germ cell-specific hexokin
ase transcript, HK1-sa, demonstrated the specific proteolytic removal of an
amino-terminal domain, resulting in a protein product identical to HK1-SC,
Such processing might constitute a regulatory mechanism governing the spat
ial and/or temporal expression of the protein.