Fig. 4. The 3D structure of the 16 nm mouse uroplakin particle at 10 Å resolution. (A) The top view of the 3D density map of a mouse 16 nm uroplakin particle that is contoured at the 1.5 level. The boundary of a `subunit' consisting of an inner and an outer subdomain is outlined in blue. (B) The particles as seen in a hexagonal crystalline array with one unit cell illustrated. (C) The side view of the 16 nm particle showing, from top to bottom, the joint (J), trunk region (TK), transmembrane domain (TM) and cytoplasmic domain (C). (D) One of the six inverted U-shaped subunits of the 16 nm particle, consisting of an inner subdomain (left) connected to an outer (right) subdomain via a (top) horizontal joint (j; outlined in blue). This inverted U-shaped subunit presumably represents a fundamental building block of the 16 nm particle (Staehelin et al., 1972; Hicks et al., 1974). (E) The inner subdomains of two neighboring subunits are connected via a minimal contact between them (double arrowhead). In A, the stellate-shaped particle has a maximal diameter of 17.5 nm and has a large lipid-filled, central hole (6 nm in diameter); in C the particle is about 12 nm tall with a 6.5 nm extracellular and a 0.5 nm cytoplasmic region [from atomic force microscopy data (Min et al., 2002)]. All panels except B are to the same scale; bar (in A) 2 nm. The unit cell in B is 16.5 nm in length.

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