The Golgi Complex: In Vitro Veritas? Review
Abstract
Understanding thestructure and function of theGolgi comiplex has proved to be among the more challenging probllems in cell biology. The last several years have turned out Ito be particularly exciting in this respect since they have Iyielded new insights and ideas at an increasingly rapid ipace. This period of advance has largely been due to the Idevelopment of powerful new biochemical, morphological, #and genetic approaches to unraveling the complexities of 'this organelle. While much remains to be discovered, the Iproblem now is how to integrate this wealth of information.
'To see if this is possible, we will first summarize how the lslolgi is commonly believed to work and then evaluate the lstrength of the evidence that underlies these views.
iplex has proved to be among the more challenging probllems in cell biology. The last several years have turned out Ito be particularly exciting in this respect since they have Iyielded new insights and ideas at an increasingly rapid ipace. This period of advance has largely been due to the Idevelopment of powerful new biochemical, morphological, #and genetic approaches to unraveling the complexities of 'this organelle. While much remains to be discovered, the Iproblem now is how to integrate this wealth of information.
'To see if this is possible, we will first summarize how the lslolgi is commonly believed to work and then evaluate the lstrength of the evidence that underlies these views.
Present View of the Golgi 'The Golgi complex is essentially a carbohydrate factory. In On the other hand, as discussed above, there is now considerable evidence in favor of several distinct "retenlion signals" that effectively prevent forward transport of membrane or lumenal proteins after reaching their prescribed destinations in the ER or various Golgi compartments (Pelham, 1991) . If this view is correct, selectivity in I.ransport through the secretory pathway may occur by "default," i.e., transport of passenger proteins proceeds owing to the absence of a retention signal. This concept, however, is not necessarily incompatible with the existence of selective signals for forward transport. It is clear that such signals exist and play an important role in directing the traffic of proteins as they leave the TGN. Examples include the mannose g-phosphate residues that specify transport of hydrolytic enzymes to lysosomes and Mellman, 1989) and the cytoplasmic domain determiinants that target newly synthesized membrane proteins to ithe basolateral surface of polarized cells (Hunziker et al., 1991a; Brewer and Roth, 1991) . Since the transport of ,fluorescent lipids from the Golgi to the plasma membrane Iin nonpolarized cells occurs very rapidly, it is thought that "constitutive" transport from the Golgi to the cell surface may not require specific signals . ,Analogous results were obtained in experiments in which a tripeptide containing a cognate site for N-linked glycosylation presumably also involved transport from the ER (Wieland et al., 1987; Helms et al., 1990) . While these experiments suggest that signals are not necessary for transport, the fact that they are released with rapid kinetics does not alone establish the absence of such signals.
Experiments showing that different secretory and membrane proteins are transported with different kinetics may indicate that signals or receptors are involved in forward transport (Lodish et al., 1983; Lodish, 1988) . Similarly, the suggestion that newly synthesized viral spike glycoproteins are present in Golgi membranes at a density severalfold greater than in the ER is also consistent with the existence of signal-driven forward transport (Griffiths et al., 1984) . However, both of these observations might also be reconciled with a nonselective mechanism of transport.
Given recent evidence that exit from the ER is linked to the folding of newly synthesized proteins, differential folding rates among proteins may indirectly affect their transport kinetics. Moreover, if the intrinsic rate of transport of glycoproteins through the Golgi is slow relative to the rate of ER exit, then one might also expect a higher concentration of certain passenger proteins in the Golgi.
At present, most of the attention paid to the question of signals in transport concerns ER to Golgi or postQolgi transport. Whether transport through the Golgi complex itself is selective or nonselective is still an open question.
After years of descriptive work, the Golgi complex is slowly starting to reveal its secrets. We have now entered an exciting period of research, during which it will become possible to define the molecular mechanisms responsible for generating and maintaining Golgi structure and function. The first phase is already well under way and has been characterized by a search for essential bits and pieces of the Golgi machinery, a number of which have already been found (NSFlsecl8, aSNAPlsecl7, (J-COP, ARF[Stearnsetal., 19901, rab6p[Goudetal., 1990] ,sec7p [Achstetter et al., 19881, and secl4p [Bankaitis et al., 199Oj) . As we have seen, however, it is at present difficult to know precisely what steps are controlled by each of these components. Nevertheless, the observed conservation of Golgi proteins between S. cerevisiae and mammals is most encouraging for our ability to confirm in living cells the function of components identified in vitro. The combination of cell-free analysis and genetics has proven its worth. The next phase will have to deal with the questions that have arisen. How many Golgi compartments are there? Are compartment boundaries defined by specific protein frameworks? If so, how do they function and how are they regulated? Does transport between Golgi compartments require vesicular carriers? What is the role of tubules? How does the machinery responsible for forward traffic relate to the machinery controlling homotypic fusion? How is specificity of forward and backward traffic regulated? How does lipid composition and organization affect transport? What function does the stack structure have? How do microtubules interact with the Golgi elements? The challenge will be to integrate the information we are now collecting in the context of how the Golgi complex works as a whole.
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