The SSU is a large ribonucleoprotein consisting of at least 40 proteins and the U3 small nucleolar RNA. It is involved in pre-rRNA processing and ribosome assembly. The SSU is necessary for the biogenesis of the 18S rRNA. Cells that are depleted of SSU proteins will arrest in the G1 phase of the cell cycle. KRR1, also known as HRB2 (HIV-1 Rev binding protein 2) or RIP-1 (Rev interacting protein 1), is a nonribosomal component of the small subunit processome (SSU). KRR1 is 381 amino acids in length and is evolutionarily conserved among human, yeast, fly, nematode and rice. KRR1 localizes to the nucleolus and is highly expressed in dividing cells. It contains one conserved KH domain (RNA-binding motif) and is a crucial component of the SSU, required for both rRNA maturation and ribosome biogenesis.
