FIGURE 1: The ribosome assembly path of Rps3.

I. The N-domain of Rps3 is co-translationally bound by the chaperone Yar1.

II. The C-domain of Rps3 dimerizes with a second Rps3 C-domain by domain swapping.

III. One copy of Yar1 is removed by competition with importin, which binds to the N-terminal NLS of one of the Rps3s.

IV. The complex is transported into the nucleus, where the importin is released.

V. One Rps3 copy is incorporated into pre-40S particles. Either after (dashed arrows) or already before Rps3 joins pre-40S, Yar1 is released and replaced by the assembly factor Ltv1. The presence of Ltv1 in pre-40S particles prevents Rps3 from attaining its final position.

VI. Presumably after export of pre-40S particles into the cytoplasm, phosphorylation of Ltv1 and contacts between the Rps3 N-domain and ribosomal protein S20 (Rps20) lead to the release of Ltv1 and the final stable incorporation of Rps3 (VII).