Gia K. Voeltz
In the Voeltz lab, we are interested in how membrane-bound organelles are generated. Most organelles have elaborate yet conserved shapes, which require the structural organization of the membrane bilayer along with its unique set of proteins. Generating and maintaining complex organelle morphologies requires specific proteins and perhaps lipids to stabilize them. It has long been clear that a complex interplay of factors must determine organelle morphology, but how, let alone the proteins responsible, are just starting to be discovered.
We are interested in understanding how the endoplasmic reticulum (ER) is formed. It is a large continuous and singular organelle with many different functions and an elaborate shape made up of several structurally distinct domains. Subdomains of the ER include the nuclear envelope (NE) and an extensive network of tubules and sheets/cisternae found in the peripheral ER (see Figure 1). We recently identified two conserved classes of integral membrane proteins, reticulons and DP1/Yop1, that generate the shape of the ER in eukaryotic cells. Some of the questions that we find most intriguing about ER biogenesis are (1) how can the membrane bilayer be shaped into different structures like tubules, sheets, and stacked cisternae when they are all continuous with each other (2) how is the ER disassembled and reassembled into the correct structure following the cell cycle, and (3) what effect does ER shape have on differentiated and polarized cell morphologies?
There are three main projects that we are pursuing in the Voeltz lab that will increase our understanding of the mechanism underlying the biogenesis of this organelle. For this work, we use three model systems including a Xenopus in vitro system for ER formation (see Figure 2), yeast cells, and mammalian tissue culture cells
Fig. 1. In vitro system for ER tubule formation from Xenopus egg extracts
In the first project, we are performing structure/function studies on reticulon and Dp1/Yop1 proteins to identify the mechanism by which they shape membrane tubules. The high degree of conservation for tubule diameter, (~ 60-100nm in yeast and vertebrates) suggests that a rigid structure may limit this dimension of ER shape. We are using sucrose gradients, FRAP, confocal fluorescence microscopy, and EM tomography to determine how reticulons and Dp1 can generate the membrane curvature that results in membrane tubules of such limited dimensions.
In the second project, we are searching for proteins that modulate and regulated the membrane-shaping activity of the reticulon and Dp1/Yop1 proteins. This project will involve both the use of our in vitro ER formation reaction and cell culture extracts to identify interacting factors.
The third project in the lab is focused on how ER shape and reticulon/Dp1 function contribute to the differentiation of a highly polarized cell that has a need for tubular ER, the neuron. These highly asymmetric cells have a morphology that includes a cell body (soma), dendritic tree, and an axon. The axon and dendrites are fine, cable-like projections, which can extend many times the diameter of the soma in length. Because of the fine dimensions of these extensions, we predict that the formation and stabilization of tubular ER by the reticulon/DP1 proteins will be a prerequisite for axon and dendritic outgrowth. Indeed, we have found that tubular ER and reticulons/DP1 are highly enriched in neurite outgrowths of neuroblastoma cell lines (Figure 2). Our goal is to test and understand the contribution of reticulons/DP1 and tubular/smooth ER to neurite outgrowth and their functions.