Three models of protein transport within the Golgi complex. (A) Cisternal maturation model. Secretory cargo is transported in an anterograde direction along with cisternae maturation from cis-face to trans-face of the Golgi complex. Golgi resident proteins are transported in retrograde through COPI vesicles. (B) Rapid partitioning model. The Golgi stacks are distributed into two sub-domains. One is glycerophospholipids containing processing domain (white), while the other domain contains glycosphingolipids (black). Transmembrane secretory protein (green) are concentrated at export domain, while Golgi-resident proteins (red) are excluded from export domain and concentrated in processing domain. (C) Golgi tubule-mediated transport model. Each cisternae is connected by Golgi tubule which is regulated by the COPI complex. Golgi tubules are involve in the rapid bidirectional transport within the Golgi complex.