This superfamily consists of the ribosomal protein L1 found in the large (50S) subunit of the bacterial ribosome, as well as its eukaryotic homologue L10 in 60S. Structural domains within this superfamily include several L1 ribosomal proteins and homologous proteins, such as eubacterial L1, algal and plant chloroplast L1, cyanelle L1, archaebacterial L1, vertebrate L10A, yeast L1-A and L1-B.

L1 is the largest protein from the large ribosomal subunit and structurally, it consists of one alpha/beta subdomain interrupted by another alpha/beta subdomain. In the structure of the protein, two domains are connected by a hinge region that consists of two oppositely directed polypeptide chains. L1 is a two-domain protein with N- and C-termini located in domain I. L1 has a dual function as a ribosomal protein binding 23S rRNA and as a translational repressor binding its mRNA. L1 from Escherichia coli (EcoL1) mediates autogenous regulation of translation by binding to a region within the leader sequence, close to the Shine-Dalgarno sequence, of the mRNA of the L11 operon coding for ribosomal proteins L1 and L11 PMID:15659579.

This superfamily entry represents the rRNA-binding domain, also called domain I. The isolated domain I has a two-layer structure. One of the layers is formed by a four-stranded anti-parallel beta-sheet, the other contains two alpha-helices. The N-terminal alpha-helix shields the interlayer region from one side, whereas two anti-parallel beta-strands cover it from the other side. It has been shown that domain I is necessary and sufficient for specific RNA-binding by L1, due to the formation of stable complexes with the isolated domain I forms stable complexes and specific fragments of both rRNA and mRNA. Experimental evidence suggests that in protein L1 its domain I alone is sufficient for specific RNA binding, whereas domain II stabilises the L1-rRNA complex. Other studies suggest that L1 interacts with the 23S rRNA through both of its domains PMID:12514741.

PFAM:PF00687, INTERPRO:IPR023674,PMID:17962298,PMID:25664749,PMID:12037305,PMID:15659579