We take our three dimensional structure for granted. Yet were it not for our skeleton, we would probably be quite flat. Cells also have a distinctive shape thanks to their skeleton – the cytoskeleton – the greater part of which is made up of actin filaments, which are assemblies of thousands of globular actin monomers. Like any scaffolding, cytoskeletons need builders to be erected; the protein twinfilin is one such builder.
Actin is at the heart of many essential cellular processes such as endocytosis, motility, cell division, secretion and intracellular signal transduction. Actin filaments have a dynamic of their own, continuously growing on one end whilst diminishing on the other. What may sound like a kind of ‘bio-indecision’ is in fact a means of lengthening, shortening, or regenerating vital structures in a cell. And twinfilin has a direct role in these dynamics.
It is a small protein with two globular domains followed by a linear domain, and looks a little like a pair of sunglasses that have lost an arm. The globular domains are able to grab hold of the actin monomers which drop off the end of an actin filament thus sequestering the monomers in the cell’s cytoplasm. The monomers are released when they are required for a growing actin filament – though how it happens, is still unclear.
Actin is vital. And twinfilin clearly has an essential role in the fine-tuning of actin filament growth. Yet – like all cellular activities – it must be harnessed, which is why discovering such proteins are of great interest. A twinfilin mutation in Drosophila, for instance, produces undersized adults and aberrant bristle morphology. Mutations in human twinfilin would certainly bring on similar distortions… Which just goes to show, yet again, how delicate and fragile the fabric of life is.