The roles of F-actin and integrin-like proteins in hyphal tip growth.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis presents an investigation into the role of F-actin and integrin-like proteins in tip growth in the oomycete Achlya bisexualis. Tip growth is a complex process that involves localised extension at the tips of hyphae. It has been suggested that F–actin plays a number of roles in the process; these include the provision of both resistive and protrusive forces and the delivery of vesicles to the sites of growth. Any role that includes the provision of force requires the attachment of F-actin to a structure such as the cell wall. To understand fully the role of F-actin in tip growth requires knowledge of its location within hyphae. Using a combination fixative of methylglyoxal and formaldehyde it was found that two distinct distributions were present. In around one half of hyphae examined a new structural feature, the F-actin depleted zone was observed. The remaining hyphae had complete F-actin caps. It is suggested that this depleted zone represents an area where localised tip yielding occurs due to a decrease in the resistive force provided by F-actin. It may also represent an area where exocytosis of wall vesicles occurs. The attachment of F-actin to the cell wall may occur via proteins that are similar to the integrins of animal cells. These proteins are sensitive to peptides that contain the sequence Arg-Gly-Asp (RGDS) and thus such peptides have often been used to indicate the presence of integrins or integrin-like proteins. The peptide Arg-Gly-Asp-Ser (RGDS) was found to reversibly affect the shape of growing hyphal tip, slow the growth rate and affect the organisation of F -actin in a dose dependent manner. Furthermore, it affected the patterns of protoplasmic retraction that were observed as hyphae were plasmolysed. This latter observation suggests that integrin-like proteins may be responsible for wall membrane attachment. These attachments are sites where F-actin is present and thus the integrin-like proteins could provide an attachment site for F-actin that allows roles in tip growth that include the provision of, or resistance to, force. Furthermore, using the technique of fractal geometry, RGDS, despite slowing growth, was found to have no effect on the morphology at the edges of mycelia. Agents that slow growth typically increase branching and thus affect such morphologies. This suggests that integrin-like proteins may play a role in coupling the processes of tip growth and branching or they may inhibit branching and thus negate the normally increased rate that is observed with the slowing of growth.