Fig. 5. Cross-sections of the 16 nm mouse uroplakin particle and placement of the transmembrane helices. (A) A 16 nm particle with the positions of the six sections (20 Å) shown in C-H (top to bottom) indicated by the horizontal lines. (B) A vertical section of one outer subdomain. A tunnel is present that appears to traverse through the entire outer domain (arrowheads in B,E-H). In F, the minimal link between the inner subdomains of neighboring subunits (arrow). (I) Five transmembrane helices, each represented by a 1 nm circle, can fit in the middle of the transmembrane zone (as shown in H). Although F-H seem to indicate the presence of a tunnel also in the inner subdomain, this tunnel does not appear to traverse through the entire height of the subdomain. (J) Hypothetical assignment of uroplakin pairs Ia/II and Ib/III to the inner and outer subdomains, respectively, of the 16 nm urothelial particle. The outlines of the 16 nm particle are drawn in thin blue lines in C-J.