Structure of the Sec23p/24p and Sec13p/31p complexes of COPII

Sterols block binding of COPII proteins to SCAP, thereby controlling SCAP sorting in ER

Sterols inhibit their own synthesis in mammalian cells by blocking the vesicular endoplasmic reticulum-to-Golgi transport of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP), a sterol-sensing protein that escorts SREBPs. Unable to reach the Golgi, SREBPs are not processed by Golgi-resident proteases, and they fail to activate genes required for cholesterol synthesis. The current studies were designed to reveal whether sterols block SCAP movement by inhibiting synthesis of special vesicles dedicated to SCAP, or whether sterols block SCAP incorporation into common coat protein (COP)II-coated vesicles. Through immunoisolation, we show that SCAP-containing vesicles, formed in vitro, also contain vesicular stomatitis virus glycoprotein (VSVG) protein, a classic marker of COPII-coated vesicles. Sterols selectively block incorporation of SCAP into these vesicles without blocking incorporation of VSVG protein. We show that the mammalian vesicular budding reaction can be reconstituted by recombinant yeast COPII proteins that support incorporation of SCAP as well as VSVG into vesicles. Sterols block SCAP incorporation into vesicles by blocking Sar1-dependent binding of the COPII proteins Sec 23/24 to SCAP. These studies demonstrate feedback control of a biosynthetic pathway by the regulated binding of COPII proteins to an endoplasmic reticulum-to-Golgi transport protein.

Cargo selection in vesicular transport: the making and breaking of a coat

Intracellular traffic is mediated by vesicular/tubular carriers. The carriers are formed by the activity of cytosolic coat proteins that are recruited to their target membranes and deform these membranes into buds and vesicles. Specific interactions between recruited coat subunits and short peptide sequences (transport motifs) on cargo proteins direct the incorporation of cargo into budded vesicles. Here, we focus on cargo selection reactions mediated by COPII and AP-2/clathrin vesicle coat complexes to explore common mechanisms by which coat assembly support localized and selective cargo sorting. Recent findings suggest that multiple, low-affinity interactions are employed in a cooperative manner to support coat assembly and enable cargo recognition. Thus low-binding affinities between coat subunits and transport motifs are transiently transformed into high-avidity, multivalent and selective interactions at vesicle bud sites. The temporal and regulated nature of the interactions provide the key to cargo selection.

COPII-dependent transport from the endoplasmic reticulum

The coat protein complex II (COPII) forms transport vesicles from the endoplasmic reticulum and segregates biosynthetic cargo from ER-resident proteins. Recent high-resolution structural studies on individual COPII subunits and on the polymerized coat reveal the molecular architecture of COPII vesicles. Other advances have shown that integral membrane accessory proteins act with the COPII coat to collect specific cargo molecules into ER-derived transport vesicles.

Surface structure of the COPII-coated vesicle

The spatial arrangement of COPII coat protein subunits was analyzed by crosslinking to an artificial membrane surface and by electron microscopy of coat proteins and coated vesicle surfaces. The efficiency of COPII subunit crosslinking to phospholipids declined in order of protein recruitment to the coat: Sar1p > Sec23/24p >> Sec13/31p. Deep-etch rotary shadowing and electron microscopy were used to explore the COPII subunit structure with isolated proteins and coated vesicles. Sec23/24 resembles a bow tie, and Sec13/31p contains terminal bilobed globular structures bordering a central rod. The surface structure of COPII vesicles revealed a coat built with polygonal units. The length of the side of the hexagonal/pentagonal units is close to the dimension of the central rod-like segment of Sec13/31. Partially uncoated profiles revealed strands of Sec13/31p stripped from the vesicle surface. We conclude that the coat subunits form layers displaced from the membrane surface in reverse order of addition to the coat.