Retro COPs

HEADLINES

Retro Coatomer

Coatomer

is essential

COSSON, interaction with

for

The existence of a retrograde transport pathway from the Colgi complex to the endoplasmic reticulum (ER), and signal-mediated retrieval of ER proteins, is well established, but so far the components of the retrograde transport machinery have remained largely obscure. Letourneur and colleagues have used the di-lysine (KKXX) retrieval signal found on certain ER membrane proteins to investigate the retrograde transport mechanism. Initially, Cosson and Letourneur showed using an in vitro system that immobilized fusion proteins containing the di-lysine motif specifically bound to proteins from yeast and mammalian cell lysates. Characterization of these proteins revealed that they were COPS: these are transport

retrieval

COPS

P. and LETOURNEUR, F. (1994) di-lysine endoplasmic reticulum Science 263, 1629-l 631

LETOURNEUR, F. et al. (1994) of dilysine-tagged proteins Cell 79, 1199-l 207

to the

vesicle coat proteins originally identified in the context of anterograde transport through the Colgi complex. The second paper describes an elegant genetic approach to isolate yeast mutants defective in retention of KKXX proteins. This yielded mutants in the a-COP and ?/-COP genes. In addition, coatomer (the complex formed by COPS) isolated from either an a-COP or p’-COP mutant, although interestingly not a ?-COP mutant, was defective in KKXX binding in in vitro assays. Thus, two separate lines of evidence lead to the conclusion that COP proteins play an important role in retrieval of KKXX proteins and interact with the di-lysine retention signal. Discovery of this new role of COP proteins is a major advance in the

retention

motifs

endoplasmic

reticulum

understanding of retrograde transport. It also adds further complexity to the apparent role of coatomer in vesicular traffic. Evidence from a variety of sources has implicated COPS in both ER-to-Colgi and intraGolgi transport, with specificity presumably being determined by additional molecules for each of the transport steps. However, these new results have prompted speculation that the main role of coatomer is in retro-grade transport, and that forward transport effects are simply a result of inhibition of the retrograde pathway. Further investigation will undoubtedly answer the very interesting question of how COPS are actually involved in each of these transport steps.

fr= Big meet Direct

evidence

that

KOCH, W. J., HAWES, G-coupled receptor Proc.

The activation of MAP kinase (MAPK) in response to agonists that stimulate receptors coupled to the heterotrimeric G protein Gi is thought to occur via Ras in some cell types, but the mechanism is not known. The work described in this paper investigated the possible role of the By subunits of Gi in MAPK activation, by utilizing cell lines overproducing the C-terminus of the B-adrenergic receptor kinase 1 (BARKI); this acts as a specific inhibitor of By-, but not OL-, mediated signalling.

1.52

small

B. E., ALLEN, L. F. and LEFKOWITZ, R. J. (1994) stimulation of mitogen-activated protein kinase activation of p21 rOs Nat/ Acad. SC!. USA 91, 12706-I 2710

Production of BARK1 in Rat-l fibroblasts abolished activation of Ras and MAPK by lysophosphatidic acid, whose receptor transduces signals through G, but not by EGF, which acts via a tyrosine kinase receptor. A similar effect of BARK1 on G,-mediated signalling, but not G,-mediated signalling, was seen in COS cells transiently expressing Gi- and G,-coupled receptors. Thus, in these cell types, Ras-dependent activation of MAPK through Gicoupled receptors is mediated via the By complex. An intriguing hypothesis TRENDS

is mediated

by Gay

proposed by the authors for the underlying mechanism is that the B-y subunits of Gi recruit and stimulate a protein that directly activates Ras. Such a protein might contain the pleckstrin homology (PH) domain implicated in binding to By complexes - and, in addition to BARK1 itself, a number of direct up- or downmodulators of Ras possess PH domains, including RasGAP, RasGRF and SOS. Exploring the interconnections between signalling by large and small G proteins opens up a new field of exciting investigations. IN CELL BIOLOGY

VOL.

5 APRIL 1995