Eukaryotic ribosomes are composed of rRNAs and ribosomal proteins. Ribosomal proteins are translated in the cytoplasm and imported into the nucleus for assembly with the rRNAs. It has been shown that chaperones or karyopherins responsible for import can maintain the stability of ribosomal proteins by neutralizing unfavorable positive charges and thus facilitate their transports. Among 79 ribosomal proteins in yeast, only a few are identified with specific chaperones. Besides the classic role in maintaining protein stability, chaperones have additional roles in transport, chaperoning the assembly site, and dissociation of ribosomal proteins from karyopherins. Bcp1 has been shown to be necessary for the export of Mss4, a phosphatidylinositol 4-phosphate 5-kinase, and required for ribosome biogenesis. However, its specific function in ribosome biogenesis has not been described. Here, we show that Bcp1 dissociates Rpl23 from the karyopherins and associates with Rpl23 afterward. Loss of Bcp1 causes instability of Rpl23 and deficiency of 60S subunits. In summary, Bcp1 is a novel 60S biogenesis factor via chaperoning Rpl23 in the nucleus.
The association between hypoechoic hepatomegaly in the third trimester and transient abnormal myelopoiesis (TAM) was reported previously in six fetuses with trisomy 21 (T21). We report a series of three cases of T21 in which hypoechoic liver (HL) was found in the second trimester but without evidence of TAM on both hematological and histological examination. We postulate that the hypo-echogenicity may be due to liver congestion secondary to hemodynamic disturbances seen in T21 fetuses. All three cases had negative first trimester Down syndrome screening and one case was detected solely because of the isolated finding of HL. HL per se may be associated with T21 and more positive cases are required to support this association.