The ability of adhesion receptors to transmit biochemical signals and mechanical force across cell membranes depends on interactions with the actin cytoskeleton. Filamins are large, actin-crosslinking proteins that connect multiple transmembrane and signaling proteins to the cytoskeleton. Here, we describe the high-resolution structure of an interface between filamin A and an integrin adhesion receptor. When bound, the integrin beta cytoplasmic tail forms an extended beta strand that interacts with beta strands C and D of the filamin immunoglobulin-like domain (IgFLN) 21. This interface is common to many integrins, and we suggest it is a prototype for other IgFLN domain interactions. Notably, the structurally defined filamin binding site overlaps with that of the integrin-regulator talin, and these proteins compete for binding to integrin tails, allowing integrin-filamin interactions to impact talin-dependent integrin activation. Phosphothreonine-mimicking mutations inhibit filamin, but not talin, binding, indicating that kinases may modulate this competition and provide additional means to control integrin functions.

The organization of secretory traffic remains unclear, mainly because of the complex structure and dynamics of the secretory pathway. We have thus studied a simplified system, a single synchronized traffic wave crossing an individual Golgi stack, using electron tomography. Endoplasmic-reticulum-to-Golgi carriers join the stack by fusing with cis cisternae and induce the formation of intercisternal tubules, through which they redistribute their contents throughout the stack. These tubules seem to be pervious to Golgi enzymes, whereas Golgi vesicles are depleted of both enzymes and cargo. Cargo then traverses the stack without leaving the cisternal lumen. When cargo exits the stack, intercisternal connections disappear. These findings provide a new view of secretory traffic that includes dynamic intercompartment continuities as key players.

Newly synthesized procollagen type I (PC) assembles into 300 nm rigid, rod-like triple helices in the lumen of the endoplasmic reticulum. This oligomeric complex moves to the Golgi and forms large electron-dense aggregates. We have monitored the transport of PC along the secretory pathway. We show that PC moves across the Golgi stacks without ever leaving the lumen of the Golgi cisternae. During transport from the endoplasmic reticulum to the Golgi, PC is found within tubular-saccular structures greater than 300 nm in length. Thus, supermolecular cargoes such as PC do not utilize the conventional vesicle-mediated transport to traverse the Golgi stacks. Our results imply that PC moves in the anterograde direction across the Golgi complex by a process involving progressive maturation of Golgi cisternae.

. Molecular proximity of seprase and the urokinase-type plasminogen activator receptor on malignant melanoma cell membranes: dependence on beta1 integrins and the cytoskeleton. Carcinogenesis 23, 1593-1601.