ER domain ribosome density without Rtns/Yop1. (A) 2D tomograph of a mutant cell with a 596-nm bud (left). Ribosomes are black dots. Note that the expansive pmaER membrane lacks tubules and fenestrations. (right) 3D model of ER domain organization in the mutant at two different angles. Peripheral ER domains are marked as in Fig. 1 with bound ribosomes indicated as dots in the color of the domain to which they are bound. (B) As in A for a mutant with a 1,253-nm bud diameter. (C) The number of ribosomes per surface area was calculated for each domain in the mother of a mutant and compared with wt cells; bud size is shown on the bottom. Domains are color coded as before with the cytoplasmic face (cyto) of pmaER in blue and PM face of pmaER in red. n = 6,511 mother ribosomes (wt = 2,044 and mutant = 4,467). (D) As in C for the bud. n = 2,029 bud ribosomes (wt = 1,727 and mutant = 302). Mut, mutant. Bars, 200 nm.