Gas Vesicles: An Introduction

Have you ever seen an algal bloom — a noxious mass of cyanobacteria floating on the surface of eutrophic ponds and lakes? These cyanobacteria, like many other aquatic microorganisms, synthesize gas vesicles to help them float to the surface. Gas vesicles are hollow, gas-filled organelles that reduce the overall density of the cell and make it buoyant enough to float in water. The synthesis and degradation of gas vesicles can be controlled by the cell to adjust its buoyancy and change its vertical position in the water column — a useful ability when competing for sunlight to photosynthesize!

Gas vesicles are found in a huge diversity of aquatic microorganisms, from green sulfur bacteria and cyanobacteria to methanogens and haloarchaea, indicating an ancient evolutionary lineage stretching back in time more than three billion years, when these two domains of life — bacteria and archaea — diverged from a common ancestor, an age when life was confined to Earth's oceans.

Structure of Gas Vesicles

Though the term “vesicle” conjures up images of phospholipid bilayers, gas vesicles are unique: they are composed entirely of protein! Shaped like a cylinder capped by cones, these protein nanostructures consist of “ribs” made of gas vesicle protein A (GvpA), a small, hydrophobic protein that is conserved across all gas-vacuolate species. By forming linear crystalline arrays — the “ribs” — GvpA monomers become the major structural units of gas vesicles.

Another gas vesicle protein, GvpC, is often bound to the external surface of gas vesicles: by interlocking between the GvpA “ribs”, GvpC strengthens the gas vesicles and increases their critical collapse pressure dramatically. However, gvpC is not an essential part of the gas vesicle gene cluster and several organisms synthesize gas vesicles lacking GvpC.

Biogenesis

One major question remains: the biogenesis of gas vesicles. How do gas vesicles form? How are they filled with gas? Current evidence suggests that gas vesicle polymerization begins from a bicone stage which gradually extends in length to form the cone-capped cylinder structure. Lengths and widths of gas vesicles vary dramatically between species — these dimensions affect the mechanical strength of gas vesicles — but remain fairly constant between cells of the same species. Electron micrography reveals that any gas-vacuolate cell includes gas vesicles at all stages of growth.

As to how the gas vesicles earn their name, the answer is surprising: surface tension at the hydrophobic inner surface of the gas vesicle (remember, GvpA is a very hydrophobic protein) excludes water molecules from the interior and only allows the free diffusion of gases through the vesicle walls! As a result, no special mechanism is required to fill up the gas vesicle; by virtue of their biochemical composition, they fill with gas even as they are synthesized.

Bioengineering Gas Vesicles

In recent years, labs around the world have bioengineered gas vesicles for diverse purposes — from ultrasonic molecular imaging to vaccine delivery — and this versatility comes from one simple structural feature of the gas vesicle: GvpC. Expressed on the gas vesicle surface, GvpC is a hydrophilic protein that accommodates insertions near its C-terminus, enabling proteins of interest to be fused to it. Antigenic peptides, fluorescent proteins and even enzymes have been successfully expressed on the surface of gas vesicles by fusion to GvpC!

This outcome can be accomplished by two distinct methods, in vivo and in vitro. The in vivo method relies on genetically modifying the gvpC gene of gas-vacuolated microbes and culturing these GMOs to produce modified gas vesicles with the GvpC-fused protein of interest expressed on the surface.

The method involves chemically stripping native GvpC off the surface of the gas vesicles, leaving only the GvpA shell, and refolding recombinantly-produced GvpC-fused protein of interest back onto the stripped gas vesicle surface.

Isolation of Gas Vesicles

The Problem

Model

Aggregating Gas Vesicles

Assaying Gas Vesicles

Future Applications of iFLOAT-enhanced Gas Vesicles

References