Cell News 3/2013
20
The cell cortex – interplay between plasma membrane
and cytoskeleton in cellular morphogenesis
Roland Wedlich-Söldner
Introduction
The plasma membrane (PM) as interface between a cell and its
environment is a prerequisite for life as we know it. As high-
ly selective barrier the PM mediates all transport of molecules
and information in an out of the cell (Figure 1). It also serves as
important platform for a multitude of signaling complexes. To
efficiently coordinate the many functions of the PM, hundreds of
lipids and proteins have to be correctly delivered and segregated
within the membrane bilayer. Due to its essential roles in cellular
homeostasis and as a signal-processing hub, the PM constitutes
one of the most important targets for pharmaceuti-cal drugs.
Perturbations in PM organization and/or function frequently re-
sult in cell death or contribute to the development of diseases,
including stroke
1
and cancer
2,3
. A detailed understanding of the
mechanisms for plasma membrane organization is therefore not
only crucial for basic cell biology but also for the development of
novel disease treatments.
Lateral membrane segregation and the cell cortex
It is now widely accepted that lipid and protein components are
not homogenously distributed within the plane of the PM - rather
they are often organized into specific nano- or micrometer-sized
domains. Several models have been advanced to explain the
emergence of lateral heterogeneities in membranes
4
. Studies
on artificial membranes have suggested generic mechanisms of
self-organization that depend upon weak interactions between
proteins and lipids
5,6
. A prominent extension of this concept is
the lipid-raft theory
7
, which postulates an – arguably simplistic
– separation of liquid ordered domains enriched in cholesterol
and sphingolipids (rafts) from liquid disordered domains mainly
containing phospholipids
8
. Most lipid-driven domains are dyna-
mic and rather small (<100 nm). In contrast, larger and more
stable domains are formed in many cells through interactions of
PM proteins with macromolecular scaffolds or membrane asso-
ciated fences. Direct protein-protein interactions
9,10
, the extra-
cellular matrix
11
/cell wall
12
and the cortical cytoskeleton
13,14
have
been shown to influence PM organization. Due to their intimate
physical and regulatory interactions, the PM and the cortical
cytoskeleton can even be considered a joint cellular structure -
the cell cortex (Figure 1). In the past years we have studied the
mechanisms and biological roles of cell cortex organization. We
have established Total Internal Reflection Fluorescence micro-
scopy (TIRFM) as a power-ful technique for the study of cortical
events in cells surrounded by cell walls, such as the model or-
ganisms
Saccharomyces cerevisiae
15,16
and Bacillus subtilis
16,17
.
We have found that rather than interfering with penetration of
the evanescent field the cell wall instead acts as light guide
4,16
,
allowing us to observe more than a third of the cell surface with
high signal to noise ratio while minimizing bleaching effects.
We have also optimized a 2D-deconvolution algorithm to further
increase contrast in our TIRFM images15.
Here I will discuss the different processes that influence cell cor-
tex organization. I will also illustrate the utility of a systems
approach when trying to understand
complex problems in cellular morpho-
genesis.
PM organization in yeast
Protein segregation in the PM of the
budding yeast Saccharomyces cerevi-
siae has been well documented. Sever-
al amino acid permeases, including the
arginine permease Can1 cluster in fur-
rowlike membrane invaginations ter-
med MCC
18
(membrane compartment
occupied by Can1), that are immobile
and tightly associated with eisosome
Figure 1. The cell cortex – a hub for
cellular organization.
Various factors influencing cell cortex
organization from outside and inside the
cell, as well as some of the diverse biological
functions and processes initiated from the
cell cortex are indicated.
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