Crystal structure of hypothetical protein TTHB192 from Thermus thermophilus HB8 reveals a new protein family with an RNA recognition motif‐like domain

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Abstract
We have determined the crystal structure of hypothetical protein TTHB192 from Thermus thermophilus HB8 at 1.9 Å resolution. This protein is a member of the Escherichia coli ygcH sequence family, which contains ∼15 sequence homologs of bacterial origin. These homologs have a high isoelectric point. The crystal structure reveals that TTHB192 consists of two independently folded domains, and that each domain exhibits a ferredoxin‐like fold with a four‐stranded antiparallel β‐sheet packed on one side by α‐helices. These two tandem domains face each other to generate a β‐sheet platform. TTHB192 displays overall structural similarity to Sex‐lethal protein and poly(A)‐binding protein fragments. These proteins have RNA binding activity which is supported by a β‐sheet platform formed by two tandem repeats of an RNA recognition motif domain with signature sequence motifs on the β‐sheet surface. Although TTHB192 does not have the same signature sequence motif as the RNA recognition motif domain, the presence of an evolutionarily conserved basic patch on the β‐sheet platform could be functionally relevant for nucleic acid‐binding. This report shows that TTHB192 and its sequence homologs adopt an RNA recognition motif‐like domain and provides the first testable functional hypothesis for this protein family.