Fission Yeast Leucine-Rich Repeat Protein Lrp1 Is Essential for Cell Morphogenesis as a Component of the Morphogenesis Orb6 Network (MOR)

In eukaryotes, cell morphogenesis is regulated coordinately with the cell cycle. In fission yeast, the morphogenesis network MOR (morphogenesis Orb6 network) consists of 5 conserved proteins, Pmo25, Nak1, Mor2, Orb6, and Mob2, and is essential for cell polarity control and cell separation following cytokinesis. Here we show that the conserved leucine-rich repeat protein Lrp1 is required for cell morphogenesis as a newly recognized component of MOR. Lrp1 has 4 leucine-rich repeats in its N-terminus and is a homolog of the budding yeast Sog2, which is a component of the RAM network (regulation of Ace2 activity and cellular morphogenesis). Lrp1 was essential for both cell growth and cell morphogenesis as were the other MOR components. Lrp1 was localized to the SPBs (spindle pole bodies, the yeast equivalent of the animal centrosome) throughout the cell cycle and to the medial ring during cytokinesis. Lrp1 interacted with Nak1 and was important for Orb6 kinase activity. Thus Lrp1 proved to function upstream of Orb6 in cell morphogenesis.