Putative role of COPI-coated vesicles and tubules according to vesicular transport and cisternal maturation models for intra-Golgi transport. According to vesicular transport model, cisternae are static. Anterograde transport may be mediated by COPI-coated vesicles and tubular cisternal connections. Another type of COPI-coated vesicles mediates retrograde transport (i.e., the retrieval of proteins that have escaped from the endoplasmic reticulum). According to cisternal maturation model, cisternae move from cis (a) to trans Golgi sides (d). COPI-coated vesicles and transient tubules may be involved in the recycling of Golgi resident proteins (retrograde transport).