The Lateral Organization and Mobility of Plasma Membrane Components

Abstract

Over the last several decades, an impressive array of advanced microscopic and analytical tools, such as single-particle tracking and nanoscopic fluorescence correlation spectroscopy, has been applied to characterize the lateral organization and mobility of components in the plasma membrane. Such analysis can tell researchers about the local dynamic composition and structure of membranes and is important for predicting the outcome of membrane-based reactions. However, owing to the unresolved complexity of the membrane and the structures peripheral to it, identification of the detailed molecular origin of the interactions that regulate the organization and mobility of the membrane has not proceeded quickly. This Perspective presents an overview of how cell-surface structure may give rise to the types of lateral mobility that are observed and some potentially fruitful future directions to elucidate the architecture of these structures in more molecular detail.

Keywords: actin cortex; lateral mobility; membrane dynamics; membrane proteins; pericellular matrix; plasma membrane; rafts.

Figure 1.. The Plasma Membrane

The cell surface defined as the plasma membrane and associated peripheral structures: schematic cross-sectional view of the plasma membrane, the subjacent membrane skeleton fence and the associated actin cortex, and the pericellular matrix. Components labeled by letters include: A, collagen. B, cross-shaped laminin. C, proteoglycan. D, fibronectin. E, integrin. F. ion channel. G, single pass transmembrane protein. H, transmembrane protein dimers. I, integral membrane proteoglycan J, GCPR. K, GPI-AP. L, M and N, adaptor proteins. O, spectrin. P, F actin. Q, MT. R, non-muscle myosin. This figure has been inspired and adapted, in part, from the drawings of David Goodsell (The Machinery of Life, second edition, Copernicus Books, NY).

Figure 2.. Modes of lateral mobility in the plasma membrane of live cells.

(A) Single particle tracking measurements in live cells indicate that the lateral mobility in the plasma membrane is heterogeneous. This is exemplified by the experimental trajectories in the plasma membrane of a live NIH 3T3 fibroblast of the sphingolipid G_M1, (red); CD59 (green), and the transmembrane protein epidermal growth factor receptor (EGFR) (blue). These molecules were simultaneously labelled, with distinct, differently emitting Qdot conjugates and imaged as described previously (Clausen et al., 2014). This data set, from a portion of the cell whose edges are indicated in a black solid line, includes apparent examples of (a) free diffusion, (b) confined diffusion, (c) channeled diffusion, (d) directed motion, and (e) diffusion interrupted by periods of transient anchorage (STALL) (Bar = 5 μm). (B) Different modes of lateral mobility can be quantitatively differentiated in traditional MSD versus time plots and subsequent analysis by curve fitting to a range of diffusion models. Shown are the cases for Brownian (free) diffusion (black); directed flow (MSD is proportional to t²; red); anomalous sub diffusion (blue); effectively 1-dimensional channeled diffusion (purple); transiently confined diffusion (green), and the extreme case of totally confined diffusion (yellow). (C) Time ranges over which different modes of lateral mobility can be observed in relation to physiological processes occurring at the plasma membrane. For flow (directed motion), the lower limit of time range is estimated for a particle with a velocity of ~2.5 μm/s (Liu et al, 2017) to move a detectable 500nm; the upper limit is for a particle undergoing retrograde flow at 1μm/min for a distance of 10μm (as in capping of antibody cross-linked antigens). The rectangular boxes at the top indicate order of magnitude time ranges for signal transduction events across the plasma membrane and for endo-and exocytotic events at membrane spanning the range from receptor mediated endocytosis to phagocytosis. For most of these phenomena, lateral diffusion (e.g. soluble membrane ligand finding membrane receptor or membrane receptor moving to site of endocytosis) is an obligatory, but not necessarily a rate limiting, step.

Figure 3.. Simulated trajectories for various modes of lateral mobility.

(A) ‘free’ diffusion, (B) weak hop diffusion, (C) strong hop diffusion, (D) diffusion interrupted by periods of transient anchorage (STALL; dashed circles), (E) channeled diffusion, and (F) directed motion. All trajectories were simulated in Mathematica for the specified sampling frequencies without positional uncertainty error. Parameters (as noted in the Figure) are chosen to resemble various examples from literature for cases of hop diffusion (Kusumi et al., 2012; Lagerholm et al., 2017), transient anchorage/STALL (with anchorage times ~2s) (Chen et al., 2009; Suzuki et al., 2007a), channeled diffusion (Jaqaman et al., 2011), and directed motion (Liu et al., 2017). Scale bars as indicated; time evolution of trajectory indicated by color bars.

Figure 4.. The cortical cytoskeleton and its effects on mobility.

A. Bleb-like protrusions on the surface of CHO cells in the process of spreading on a substrate as imaged by scanning electron microscopy. B. How blebs and/or bleb-like protrusions could compartmentalize lateral diffusion (see text for discussion). The labels in panel B are: A, ion channel; B, single pass transmembrane protein; C, GCPR; D, GPI-AP; E, transmembrane protein dimer; F, F-actin; G, spectrin; H, non-muscle myosin. C,D. How rapid, MT-based directed transport might occur in the presence of the membrane skeleton fence (see text for discussion). C. Cortex deforms to allow MT and its associated motor(s) direct access to PM protein cluster. The labels in panel C are: A-D are the same as in panel B; E, oligomeric transmembrane protein, such as DC-SIGN, coupled to a MT motor; F, transmembrane protein dimer; G, adaptor protein, H, motor protein; I, MT; J, F-actin; K, spectrin. D. MT and associated motor (s) proximate to the inner leaflet is able to slice through the membrane skeleton fence due to motor force and thermally activated breaks in the fence. The labels in panel D are: A-K are the same as in panel C. Panels B,C and D have been inspired and adapted, in part, from the drawings of David Goodsell (The Machinery of Life, second edition, Copernicus Books, NY)