English: Nucleus is gray; SPB is black circle; MTs are black bars; actin filaments are gray cables; actin patches are small gray circles. (A) Nuclear orientation to the shmoo tip. MTs are guided along filamentous actin toward the shmoo tip. Kar9p, Bim1p, and Myo2p are required for nuclear orientation, but the contributions of Kar3p, Bik1p, and Kip2p are unknown. (B) MT attachment to the shmoo tip. MTs are tethered to the mating projection by Kar3p during depolymerization and Bim1p during polymerization. Bik1p and Kip2p function in MT attachment is unknown. (C) Before cell–cell fusion, MTs are maintained at the shmoo tip. (D) Sliding cross-bridge model for nuclear congression. Oppositely oriented MTs overlap and are cross-linked along their lengths, whereas depolymerization is induced at the spindle poles (Rose, 1996). (E) Plus end model for nuclear congression. MT plus ends cross-link and induce depolymerization to draw opposing nuclei together. In either the sliding cross-bridge or plus end models, the localization and/or function of Kar3p, Bik1p, Kip2p, and Kar9p during live cell nuclear congression is not known.