An Analysis of Woronin Body Biogenesis in Neurospora Crassa

Abstract

Eukaryotic organelles evolve to support the lifestyle of evolutionarily related organisms. In the fungi, filamentous Ascomycetes possess dense-core organelles called Woronin bodies (WBs). These organelles originate from peroxisomes and perform an adaptive function to seal septal pores in response to cellular wounding.

Previously, it was found that WBs are centered on a self-assembled HEX1 protein in N. crassa. A WB specific membrane protein WSC that is required for WB biogenesis has been identified recently to envelope HEX assemblies at the peroxisome membrane and help them bud from the peroxisome matrix. Also, it is known that cortical association of WB requires WSC.

I conducted a screen for proteins associated with WB biogenesis and identified Leashin (LAH). Using genetic, cellular and biochemical methods, I show that Leashin encodes an organellar tether required for WB inheritance. In addition, I associate Leashin with evolutionary variation in the subcellular pattern of WB distribution.

In Neurospora, the leashin locus encodes two related adjacent genes. I named the 5¿ gene lah-1 and the downstream gene lah-2. The N-terminal sequences of LAH-1 bind WBs via the WB¿specific membrane protein WSC, and C-terminal sequences are required for WB inheritance by cell cortex association. LAH-2 is localized to the hyphal apex and septal pore rim and plays a role in colonial growth.

In most species, WBs are tethered directly to the pore rim. However, Neurospora and relatives have evolved a delocalized pattern of cortex association. Using a new method for the construction of chromosomally encoded fusion proteins, marker fusion tagging (MFT), I show that a LAH-1/LAH-2 fusion can reproduce the ancestral pattern in Neurospora. My results identify the link between the WB and cell cortex and suggest that splitting of leashin played a key role in the adaptive evolution of organelle localization. I present a model in which WB biogenesis occurs in a three-tier process that links organellar morphogenesis and inheritance.