Fig. 1: PQBP5 forms the skeletal structure of the nucleolus.

a Prediction of intrinsically disordered regions (IDRs) in human PQBP5, fibrillarin, and nucleolin proteins by two independent programs. The IDR in PQBP5 was predicted at the C-terminus, whereas the IDRs in fibrillarin and nucleolin were predicted at their N-termini. b High-speed AFM reveal PQBP5 protein with a globular region and a dynamically moving IDR tail of PQBP5 (upper panel, see Supplementary Movie 1), PQBP5 with a small spherical domain near the globular region interacting with RNA (middle panel, see Supplementary Movie 2) or PQBP5 with a small spherical structure transiently formed at the IDR tail and interacting with RNA (lower panel, see Supplementary Movie 3). The relationship between PQBP5-RNA contact and transient globular structure formation was assessed by two-sided Fisher’s exact test. The similar observation was repeated more than three times. c Structure of PQBP5 predicted by AlfaFold (https://alphafold.ebi.ac.uk/). d Representative super-resolution microscopy (SRM) image of HeLa cells showing the spatial relationship among PQBP5, fibrillarin, and nucleolin proteins at four nucleoli in a nucleus. DAPI shows the area of the nucleus. The nucleolus within the white line square is further shown in Fig. 1e, whereas the other three nucleoli are shown in Supplementary Figure 1. The similar observation was repeated more than ten times. e Merged image of the indicated nucleolus showing single and two-protein images. Small PQBP5 dots were distributed homogeneously throughout the nucleolus, distinguishing PQBP5 from nucleolin and fibrillarin. PQBP5 dots were in contact with nucleolin or fibrillarin dots, but did not merge with them. The similar observation was repeated more than ten times. f Analysis of signal intensities of PQBP5, nucleolin, and fibrillarin, showing their distinct distribution patterns. g 3D image of the three proteins generated by Imaris (see Supplementary Movie 4 for original data). h Correlative light and electron microscopy (CLEM) of the three nucleolar proteins. The most electron dense area corresponds to DFC stained with antibody to fibrillarin (red), whereas anti-PQBP5 antibody stained the middle electron dense skeletal structure of the nucleolus (green). The similar observation was repeated more than ten times.