Intracellular localization and in vivo trafficking of p 24 A and p 23

Transport of proteins along the secretory pathway of eukaryotic cells is mediated by vesicular carriers, which bud off from a donor compartment and fuse with an acceptor compartment (Palade, 1975). Formation of these transport vesicles is dependent on recruitment of cytosolic coat proteins on the surface of the donor compartment membrane. Two types of coat structures, COPI and COPII, have been shown to mediate the transport between the endoplasmic reticulum and the Golgi apparatus (for a review see Rothman and Wieland, 1996). Whereas the COPI coat is involved in anterograde ER to Golgi (Pepperkok et al., 1993; Peter et al., 1993; Bednarek et al., 1995), intra-Golgi (Orci et al., 1986), and retrograde Golgi to ER transport (Cosson and Letourneur, 1994; Letourneur et al., 1994), COPII seems to mediate exclusively anterograde transport from the ER to pre-Golgi structures (Barlowe et al., 1994; Aridor et al., 1995). Vesicles are not the only vehicle for ER to Golgi transport. Recently it has been shown that anterograde intermediate compartment (IC) to Golgi transport can occur by pre-Golgi carriers (Presley et al., 1997; Scales et al., 1997). These are non-vesicular membrane structures, sometimes larger than 1.5 μm in diameter, which move en masse on microtubule tracks in a ‘stop-and-go’ fashion towards the Golgi complex (Presley et al., 1997). Recently we have cloned and characterized p23 (Tmp21) and p24A from rat pancreatic microsomal membranes (Blum et al., 1996). These proteins exhibit weak homology (23% identity). They are members of the p24 protein family, which were initially described by Stamnes et al. (1995). All p24 members share the same type I topology, with a large luminal domain, followed by a C-terminal located membrane anchor a d a short highly conserved cytoplasmic tail. The function of p24 proteins is not yet known, but experiments with the yeast p24 proteins Emp24p and Erv25p indicate that they are involved in the sorting and/or concentration of specific cargo proteins in ER-derived COPII-transport vesicles (Schimmöller et al., 1995; Belden and Barlowe, 1996). p24 proteins exhibit all the features of putative cargo receptors, as suggested by Balch et al. (1994). So far, no direct or indirect interaction of p24 proteins and cargo proteins has been shown. The only mammalian p24 member for which steady-state localization has been described is p23. It is localized to Golgi cisternae (Sohn et al., 1996), is concentrated into COPI vesicles, and is able to bind COPI proteins by its cytosolic tail peptide in vitro. It therefore had been assumed to be an integral Golgi-specific receptor for the COPI coat. However, Rojo et al. (1997) showed that p23 is a major component (30% of the total protein) of intermediate compartment/cis-Golgi network membranes, is de-enriched in COPI-coated vesicles, and is not required for COPI recruitment onto donor membranes. Here we describe the intracellular localization of p24A and compare it with p23. We have localized p24A to membranes 537 Journal of Cell Science 112, 537-548 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JCS4537