Cytochrome b5 and a recombinant protein containing the cytochrome b5 hydrophobic domain spontaneously associate with the plasma membranes of cells

The membrane-interactive tail of cytochrome b(5) can function as a stop-transfer sequence in concert with a signal sequence to give inversion of protein topology in the endoplasmic reticulum

Sequence analyses of the C-terminal membrane intercalative region of the rat cytochrome b(5) indicated that this domain has, in addition to a signal sequence, a combined element of the classic stop-transfer sequence typically found in a variety of transmembrane proteins. Such bitopic protein arrangements arise by tandem but topogenically displaced activities of cleavable/noncleavable signal and stop-transfer sequences. A fusion precursor comprising an N-terminally linked prokaryotic signal sequence and the full-length of mammalian cytochrome b(5), including its C-terminal membrane insertion sequence, was engineered to investigate the outcome of this combination of signals on the targeting and topology of the cytochrome b(5) in the endoplasmic reticulum membrane. Precytochrome b(5) was cotranslationally translocated across the endoplasmic reticulum membrane. The signal-processed cytochrome b(5) was integrally anchored in the membrane with the globular domain facing the lumen. Thus, the topology of the signal sequence-directed cytochrome b(5) in the microsomal vesicle was reversed with respect to that of the native form. Posttranslational incubation of the precytochrome b(5) with microsomes resulted in a "loose" incorporation of the unprocessed form onto the surface of the vesicle. Our findings suggest that the membrane-insertion sequence of cytochrome b(5) has a functional stop-transfer sequence. We discuss the implications of these findings with respect to selective targeting of cytochrome b(5) to the endoplasmic reticulum membrane in the view that signal and stop-transfer sequences are often interchangeable or combined for topogenic functions.

Tobacco cytochrome b5: cDNA isolation, expression analysis and in vitro protein targeting

A full-length clone encoding cytochrome b5 has been isolated from a tobacco leaf cDNA library in lambda gt11 by PCR using degenerate primers. This cDNA encodes a protein of 139 residues which exhibits a high degree of homology to other cytochrome b5s, the message for which is expressed predominantly in developing seeds and in pigmented flower tissue. In the developing tobacco seed the mRNA is abundant at very early stages (< 10 days after flowering). Southern analysis indicated that more than one gene encodes cytochrome b5 in the tobacco genome. In vitro transcription and translation studies of the cDNA indicated that the protein inserts into the ER membrane by a non-SRP-mediated pathway and that the C-terminus of the protein is required for targeting and insertion.