Stop-and-go movements of plant Golgi stacks are mediated by the acto-myosin system

The Golgi apparatus in plant cells consists of a large number of independen t Golgi stack/trans-Golgi network/Golgi matrix units that appear to be rand omly distributed throughout the cytoplasm. To study the dynamic behavior of these Golgi units in living plant cells, we have cloned a cDNA from soybea n (Glycine max), Gm-Man1, encoding the resident Golgi protein alpha-1,2 man nosidase 1. The predicted protein of approximately 65 kD shows similarity o f general structure and sequence (45% identity) to class I animal and funga l alpha-1,2 mannosidases. Expression of a GmMan1::green fluorescent protein fusion construct in tobacco (Nicotiana tabacum) Bright Yellow 2 suspension -cultured cells revealed the presence of several hundred to thousands of fl uorescent spots. Immunoelectron microscopy demonstrates that these spots co rrespond to individual Golgi stacks and that the fusion protein is largely confined to the cis-side of the stacks. In living cells, the stacks carry o ut stop-and-go movements, oscillating rapidly between directed movement and random "wiggling." Directed movement (maximal velocity 4.2 mu m/s) is rela ted to cytoplasmic streaming, occurs along straight trajectories, and is de pendent upon intact actin microfilaments and myosin motors, since treatment with cytochalasin D or butanedione monoxime blocks the streaming motion. I n contrast, microtubule-disrupting drugs appear to have a small but reprodu cible stimulatory effect on streaming behavior. We present a model that pos tulates that the stop-and-go motion of Golgi-trans-Golgi network units is r egulated by "stop signals" produced by endoplasmic reticulum export sites a nd locally expanding cell wall domains to optimize endoplasmic reticulum to Golgi and Golgi to cell wall trafficking.