We have made the key discoveries that Runx2, the osteoblast maturation promoting transcription factor, controls cell cycle progression in pre-osteoblastic cells, and progression through the cell cycle modulates Runx2 activity at three different cell cycle stages (i.e., the G0/G1, G1/S and M/G1 transitions) by both transcriptional and post-transcriptional mechanisms. Runx2 performs a critical epigenetic function during mitosis and its expression in early G1 occurs when normal osteoblasts monitor extracellular cues for competency to initiate cell cycle progression. We propose that selective proteolytic degradation of Runx2 in normal osteoblasts is critical for acute downregulation of Runx2 prior to entry into S phase. The central hypothesis of our proposal is that Runx2 peptide motifs (Specific Aim 1) and ubiquitination of Runx2 (Specific Aim 2) mediate its selective degradation during late G1 in the osteoblast cell cycle to prevent cell growth suppression through activation of a putative cell cycle checkpoint. Specific Aim 1 will examine what Runx2 peptide motifs, which represent part of a putative molecular relay that stages protein/protein interactions, are required for Runx2 degradation. Specific Aim 2 will identify the E3 ubiquitin ligase that destabilizes Runx2 and characterize novel osteoblast-related co-factors (e.g., F-box proteins) that support ubiquitin ligation.