Tmp21 and p24A, Two Type I Proteins Enriched in Pancreatic Microsomal Membranes, Are Members of a Protein Family Involved in Vesicular Trafficking

doi:10.1074/jbc.271.29.17183

Tmp21 and p24A, Two Type I Proteins Enriched in Pancreatic Microsomal Membranes, Are Members of a Protein Family Involved in Vesicular Trafficking*

^‡ Institute of Physiology II,
^¶ Medical Biochemistry, University of the Saarland, 66421 Homburg Saar, Germany and
^§ Laboratorium voor Fysiologische Scheikunde, University of Gent, 9000 Gent, Belgium

^∥To whom correspondence should be addressed. Tel.: 49-(0)6841/16-6450; Fax: 49-(0)6841/16-6655.

Abstract

We report here on the isolation, cloning, and expression of two M_r 21,000 proteins from rat pancreatic acinar cells, the rat-Tmp21 (ransembrane rotein, M_r 21,000) and the rat-p24A. Both proteins are transmembrane proteins with type I topology and share weak but significant homology to one another (23% identity). We further show the cloning and characterization of the human homologs, hum-Tmp21, which is expressed in two variants (Tmp21-I and Tmp21-II), and hum-p24A. Tmp21 proteins and p24A have highly conserved COOH-terminal tails, which contain motifs related to the endoplasmic reticulum retention and retrieval consensus sequence KKXX. The rat-p24 sequence is identical to the hamster CHOp24, a recently characterized component of coatomer-coated transport vesicles, which defines a family of proteins (called the p24 family) proposed to be involved in vesicular transport processes (Stamnes, M. A., Craighead, M. W., Hoe, M. H., Lampen, N., Geromanos, S., Tempst, P., and Rothman, J. E. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 8011-8015). Sequence alignment and structural features identify the Tmp21 protein as a new member of this p24 family. Northern analysis of various tissues indicates that the Tmp21 proteins and the p24A protein are ubiquitously expressed. The integral membrane components Tmp21 and p24A are localized in microsomal membranes, zymogen granule membranes, and the plasma membrane and are absent from the cytosol. Both p24A and Tmp21 show weak homology to the yeast protein Emp24p, which recently has been shown to be involved in secretory protein transport from the endoplasmic reticulum to the Golgi apparatus. This leads us to conclude that the receptor-like Tmp21 and p24A are involved in vesicular targeting and protein transport.

Footnotes

↵* This study was supported by Grants SFB 246, B14 from the Deutsche Forschungsgemeinschaft. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

ER

endoplasmic reticulum

PAGE

polyacrylamide gel electrophoresis

Tmp

transmembrane protein

TBS

Tris-buffered saline

hum-

human

bp

base pair(s)

ORF

open reading frame

kb

kilobase(s)

SSRα

signal sequence receptor α

PM

plasma membrane.
- Received November 27, 1995.
- Revision received May 1, 1996.