Previously 2 main types of Endocytosis 3 main trafficking routes for newly internalized receptors and ligands Now: If there is traffic there must be roads Cellular infrastructure: the components of the cytoskeleton

Molecular Roads and Infrastructure Bipolar disorder Is it the cellular equivalent of ‘dead letter box’? Are cytoskeletal ‘problems’ a likely cause for for this disorder? What are the components of the cytoskeleton and what do they do?

Elements of the Cytoskeleton Intermediate filaments Microfilaments (actin) Microtubules

Actin Fig 18-4 Highly conserved Most abundant intracellular protein (in eukaryotes)

Structure of actin Fig 18-2 Polarity of molecule ‘-” and ‘+’ ends T form and D form G- actin vs F- actin

Bundles and Networks What is the benefit of these kinds of associations?

Stability of actin Stability depends on environment– ion and g-actin concentration Provides support through DYNAMIC arrangements including both structure and gel-like qualities of cytosol

Actin filaments dynamics + vs - end? Illustration of treadmilling CBI 25.2 Filaments utilizes 3 steps: lag period, elongation, steady state ATP hydrolysis NOT required for polymerization ATP hydrolysis changes kinetics of polymerization

Microtubules Fig 19-5 Fig 19-1 Protofilament: Polymer of  /  tubulin heterodimers 13 protofilaments= microtubule Stable and unstable populations Exhibit dynamic instability

Microtubule dynamics + vs - end Steps in formation protofilament formation microtubule assembly microtubule elongation Microtubules treadmill AND undergo dynamic instability

Factors effecting polymerization/ depolymerization Critical Concentration: C c Actin: ends have different C c (+) 0.1  M (-) 0.8  M cellular concentration 0.5  M Consequences? Microtubules: One end ‘in’ MTOC Consequences? Associated proteins Toxins

And why are we discussing this? Shmoos form from reorganization of actin cytoskeleton Shmoo tip ‘extends’ due to vesicles specifically delivered via actin bound motor protein Microtubules are main ‘highway’ vesicular traffic

Cytoskeleton critical for localization Fig Actin in development CBI21.5 Microtubules in axon core and protein localization Intermediate filaments in mechanical stability of neurons But how do the vesicles move?

What makes one MT different from another?Accessories!