Microscopy image of a developing egg cell of a fruit fly with one protein distributed around the entire cortex (red) and a second one located in a narrow part of the posterior cortex (green). Image credit: Bao et al. (CC BY 4.0)
Cells are brimming with many different proteins, compartments, and other cell components that all play specific roles, often at very precise locations in a cell at particular moments in time. Human cells, like those of other animals and plants, contain long tracks called microtubules that are able to transport such components to wherever they are needed.
Microtubules consist of chains of proteins known as tubulins that the cell can modify with small molecule tags at specific locations. For example, an enzyme called TTLL5 attaches molecules of glutamic acid to multiple positions on one of the tubulin proteins (known as α-tubulin). However, it remains unclear what role such modifications have on the ability of microtubules to move components around the cell.
Fruit flies are often used as models of animal biology in research studies. Three different versions of α-tubulin are found within the ovaries of fruit flies. Two of these are ‘general’ α-tubulins that are expressed in almost all tissues around the body, but the third is exclusively made in the ovaries.
Bao et al. studied the effect of TTLL5 activity on microtubules in fruit flies. The experiments revealed that TTLL5 played a crucial role in adding glutamic acid marks to the two general α-tubulin proteins. These modifications were needed for microtubules to successfully distribute a transporting motor protein named kinesin-1 to where it was needed for cargo transport within the egg cells. On the other hand, glutamic acid tags were not added to the oocyte α-tubulin protein.
Further experiments studied nerve cells, called neurons, in the wings of the flies. In mutant fruit flies with inactive TTLL5 enzymes, cell compartments known as mitochondria moved along microtubules to one end of the neurons with fewer pauses than those in normal cells.
This work shows that glutamic acid tags play important roles in regulating the transport of cell components along microtubules in fruit flies. In the future, these findings may support efforts to develop new treatments for human neurodegenerative diseases that are linked to defects in microtubules.
