S. Lin et al., STRUCTURAL REQUIREMENTS FOR INTRACELLULAR-TRANSPORT OF PULMONARY SURFACTANT PROTEIN-B (SP-B), Biochimica et biophysica acta. Molecular cell research, 1312(3), 1996, pp. 177-185
Human SP-B is synthesized by the alveolar Type II epithelial cell as a
381 amino acid preproprotein. The 79 residue mature SP-B peptide is e
xtremely hydrophobic and flanked by propeptides of 200 and 102 amino a
cids at its NH2- and COOH-termini, respectively. The purpose of this s
tudy was to identify peptide domains of the SP-B proprotein necessary
for trafficking of the mature peptide in the secretory pathway. To thi
s end several constructs were generated, by subcloning the full length
human SP-B (SP-B), COOH-terminally truncated SP-B (SP-Bh(Delta C), in
which residues 201-381 were deleted), NH2-terminally deleted SP-B (SP
-B-Delta N, in which residues 28-200 were deleted), NH2-terminal prope
ptide (SP-B-N), mature SP-B (SP-B-M) and COOH-terminal propeptide (SP-
B-C), into the mammalian expression vector pcDNA3. The resulting expre
ssion constructs were characterized by DNA sequencing and in vitro tra
nscription/translation and subsequently transfected into Chinese hamst
er ovary cells. 48 h after transfection, cells were labeled with [S-35
]-met/cys and analyzed by immunoprecipitation, SDS-PAGE and autoradiog
raphy. Proteins encoded by SP-B, SP-Bh(Delta C) SP-B-N and SP-B-C cons
tructs were secreted into media; in contrast, SP-B constructs lacking
the NH2-terminal propeptide (SP-B-Delta N) remained in the endoplasmic
reticulum (as assessed by endoglycosidase H sensitivity) and were rap
idly degraded. We conclude that (1) 27 amino acids at the NH2-terminus
of SP-B contain a functional signal peptide and (2) the NH2-terminal
propeptide of the SP-B precursor is necessary and sufficient for intra
cellular trafficking of the mature peptide.