12.1Protein Synthesis and Basis of Intracellular Transport
Membrane-bound and Free Polysomes
Fig. 12-1 Intracellular transport system in animal cells
Outline of the pathway for transport of proteins in eukaryotic cells. There are different methods for transporting protein synthesized by membrane-bound and free polysomes. ← shows the primary direction of transport.
The transport of synthesized proteins is a major part of intracellular transport. Protein synthesis in eukaryotic cells is performed by two types of polysomes*1: membrane-bound polysomes that are bound to the endoplasmic reticulum (ER) and free polysomes that are separate and free within the cell (Fig. 12-1). The former synthesizes membrane and secretory proteins. After these proteins have been synthesized in ER, they are transported to the cell membrane or endosome (see Chapter 6) through the Golgi body. Specialized transport systems function at this time. Free polysomes synthesize proteins required by organelles such as the nucleus, mitochondria, chloroplasts, and peroxisome, as well as enzymes and structural proteins that function within the cytoplasm. These do not require a specialized transport system but are rather transported to target sites by diffusion through the cytoplasm after synthesis by free polysomes.
*1 Protein is synthesized by multiple aggregated ribosomes on mRNA. The ribosomal aggregate is called a polysome or polyribosome (see Selection 4 of Chapter 9).
Fig. 12-2 Transporting substances between membrane lines using a transport vesicle
A protein synthesized in the endoplasmic reticulum (ER) is loaded onto a transport vesicle formed by the action of a coat protein. Once the coat protein has been removed, the transport vesicle is shipped to a target membrane (e.g., the Golgi body). Once bound to the target membrane, the vesicle fuses with the target membrane to deliver the cargo and membranous components inside it. The electron microscope photograph shows a transport vesicle contained in a coat protein.
Fig. 12-3 Formation of a transport vesicle
(A) Structure of clathrin constituting a coat protein and its polymerization. Clathrin is composed of three heavy chains with three light chains bound to them, forming units called triskelions. Six triskelions polymerize to form a hexahedron. (B) The G protein GDP, called ARF, interconverts to GTP and is activated by the GDP and GTP exchange factor (GEF). The coat protein binds to the membrane protein or cargo receptor to cause vesicle budding. (C) The vesicle structure is formed from a flat membrane so it can be packaged by the coat protein. When the vesicle detaches from the membrane, a transport vesicle loaded with cargo is formed.
During the series of transport processes between organelles, small-sized vesicles, called transport vesicles, with a diameter of approximately 50–150 nm are used. These transport processes consist of (1) cargo loading and vesicle budding, (2) vesicle transport, (3) binding of the vesicle to a target membrane, and (4) delivery of the cargo and membranous components by fusing the membranes of the vesicle and target membrane (Fig. 12-2).
First, the cargo aggregates near the ER membrane, and a transport vesicle is then formed (buds) when coat proteins from the cytoplasm side bind together. Three types of complexes for coat proteins are known: a complex of clathrin (Fig. 12-3A) and adaptin, coat protein I (COP I) complex, and COP II complex. Each of these complexes function along different transport routes. These coat proteins envelop the budding vesicle, thus forming a transport vesicle. Let us look at this process from the perspective of the clathrin–adaptin complex. The complex noted through this binds to the cargo receptor and leads to formation of a vesicle (Fig. 12-3B, C).
The spherical vesicle budding out from a flat membrane detaches from the membrane to become an independent transport vesicle (Fig. 12-3C). A G protein called dynamin (see Selection 2 of Chapter14 plays a role in detaching the budded vesicle. When the vesicle buds, dynamin polymerizes by winding around the root of the vesicle in a helical shape and uses the energy from GTP hydrolysis to detach the transport vesicle from the membrane.
The transport vesicle delivers the cargo to the transport destination without ever making a mistake. The mechanism underlying this transport system can be explained using a model called SNARE hypothesis*2. In this model, the transport vesicle accurately delivers the cargo to the correct transport destination by selectively binding the membrane protein v-SNARE, which is distributed throughout the membrane of the transport vesicle, to the membrane protein t-SNARE, which is distributed throughout the target membrane (Fig. 12-4). Until recently, v-SNARE and t-SNARE were only considered as unknown substances in models; however, currently, their existence has been made clear and numerous transport routes function by including different types of v-SNARE and t-SNARE.
The transport vesicle follows several steps to travel to the target membrane, which serves as the transport destination, and to deliver the cargo to its counterpart. The initial step is when the transport vesicle, which reaches the target membrane, is stopped against the target membrane by the membrane proteins. Next, the transport vesicle and target membrane are tightly bound by the SNARE proteins (with an α-helix structure). Further, the SNARE proteins become intertwined with each other such that the two membranes are tightly adhered to each other. Membrane fusion between the tightly adhered transport vesicle membrane and target membrane occurs because of an increase in the peripheral concentration of Ca2+. As a result, the cargo of the transport vesicle and membrane components of the transport vesicle are delivered to the transport destination (Fig. 12-5).
*2 SNARE is an abbreviation for the soluble N-ethylmaleimide–sensitive factor (NSF) attachment protein (SNAP) receptor. SNAP signifies proteins that bind with a protein factor called NSF (these are protein complexes that have ATP-degrading enzymatic activity). Furthermore, NSF signifies a protein that is deactivated by N-ethylmaleimide.
Fig. 12-4 The SNARE model
The two types of transport vesicles formed from ER and the cell membrane, which differ in their destinations, can accurately bind to their respective target membranes because of the selective binding between v-SNARE found on the transport vesicle and t-SNARE found on the target membrane.
Fig. 12-5 Binding and fusion of a transport vesicle onto a target membrane
After the transport vesicle has latched onto the target membrane, the specific binding between the SNARE proteins causes both of them to bind tightly to each other. Increase in the surrounding Ca2+ concentration causes the transport vesicle and target membrane to fuse their membranes together so that the cargo or membrane protein inside the transport vesicle can be delivered to its transport destination.