Serum albumin as a potential carrier for the apocrine secretion of proteins in the rat coagulating gland.
A protein of 66k was purified to homogeneity from the total secretion of rat coagulating gland. Its close structural relationship to serum albumin was demonstrated by N-terminal amino acid sequence analysis, proteolytic fingerprinting and Western blotting studies using polyclonal antibodies raised against the 66k protein and rat serum albumin. Immunofluorescence staining showed that the 66k protein was localised in the cytoplasm of coagulating gland epithelial cells from which it is released via apocrine blebs. Performing immunoelectron microscopy, the 66k protein was by no means detectable in the endoplasmic reticulum and the Golgi apparatus. Reverse transcription-PCR, Northern blotting studies and in situ hybridisation experiments demonstrated that mRNA of albumin is not expressed by coagulating gland epithelial cells. Therefore, intravascular albumin should be transferred into the epithelial cells of the rat coagulating gland followed by secretion via aposomes. Furthermore, overlay blots proved that the 66k protein binds to the apocrine proteins carbonic anhydrase II and secretory transglutaminase and vice versa. In contrast, no binding was evident to the merocrine 115k protein and to cytoplasmic resident proteins e.g. lactate dehydrogenase. These findings point to the assumption that serum albumin taken up from extracellular sources could function as a selective carrier for cytoplasmic proteins destined for apocrine secretion.
Wilhelm
B
B
Institut für Anatomie und Zellbiologie, Philipps-Universität Marburg, Germany.
Meinhardt
A
A
Renneberg
H
H
Linder
D
D
Gabius
H J
HJ
Aumüller
G
G
Seitz
J
J
D000818:Animals; D001050:Apocrine Glands / Q000502:physiology*; D001779:Blood Coagulation Factors / Q000502:physiology*; D007150:Immunohistochemistry; D008099:Liver / Q000033:anatomy & histology; D008297:Male; D051381:Rats; D017208:Rats, Wistar; D012709:Serum Albumin / Q000032:analysis / Q000502:physiology*
D001779:Blood Coagulation Factors; D012709:Serum Albumin