Cell Structure and function test 4 extra textbook notes Lecture 32 Flagellar motility in sperm tail and chamydomonas algae o Chlamydomonas reinhardtii revealed particles moving a constant speed toward tip anterograde movement and others moving from the tip of the base retrograde intraflagellar transport occurs in both flagella and cilia o Anterograde movement powered by kinesin 2 o Retrograde movement powered by cytoplasmic dynein o Cilia and flagella related microtubule based and membrane bound extensions that project o Abundant motile cilia found on surface of specific epithelia wavelike fashion movement o Animal cell flagella propel cell through liquid o Both contain many microtubule based motors axonemal dyneins beating of flagella kinesin 2 and cytoplasmic dynein are responsible for flagella and cilia assembly and turnover MT orientation in cilia flagella o Contain long doublet microtubules bridged by dynein motors o All microtubules in cilia and flagella have the same polarity ends tip attached in the cell axoneme connects with the basal body complicated structure containing 9 triplet microtubules Cross section of cilia flagellum 9 2 array of MTs inside PM o Central bundle of microtubules axoneme 9 2 arrangement of 9 doublet microtubules surrounding a central pair of single ultrastructurally distinct microtubules o Each of the 9 outer doublets consists of A microtubule filament with 13 protofilaments and B microtubule with 10 protofilaments Axoneme structure o Held together by 3 sets of protein cross links o Central pair of singlet microtubules connected by periodic bridges o Second set of linkers nexin joins adjacent outer doublet microtubules toward central pair Radial strokes project from each A tubule of outer doublets o Major motor protein present in cilia and flagella axonemal dynein large multisubunit protein related to cytoplasmic dynein o 2 rows of dynein motors attached periodically down the length of each A tubule of the outer doublet microtubules inner arm and outer arm dyneins interact with adjacent B tubule that bring about cilia and flagella bending ATP dependent sliding of outer doublets generates force for motility o Cilia and flagella motile structures because the axonemal dynein motors bending o Bend starts base of cilium flagellum propagates along structure o Axonemes treated with protease that cleaves nexin links o ATP added to treated axonemes doublet microtubules slid past one another as dynein attached to A tubule of one doublet walked down the B tubule of the adjacent doublet o Axoneme with intact nexin links action of dynein induces flagellar bending as microtubule doublets connected to one another Axonemal dynein mediated bending o Axonemal dynein attached to A tubule of outer doublet pulls on B tubule of adjacent tubule trying to move to the 0 end o Because adjacent tubules tethered by nexin force generated by dynein bends cilium flagellum o Experimental evidence when nexin links are cleaved with protease and ATP added to induce dynein activity microtubule doublets slide o Protease treated axoneme 2 doublet microtubules incubated with past each other ATP In absence of cross linking proteins doublet microtubules slide excessively dynein arms project from A tubules interacting with B tubules of left microtubule doublet Primary cilium is assembled by centrosome o Assembled from basal body o Built around 9 linked triplet microtubules o Continuous with A and B tubules of axoneme membrane bound core of cilium flagellum o Between basal body and axonome transition zone Key characteristics of IFs o Extend throughout cytoplasm o Line inner nuclear envelope of interphase animal cells o Biochemically much more heterogenous o Have great tensile strength o Don t have intrinsic polarity o No known motors that use them as tracks o Dynamic in terms of subunit exchange much more stable than microfilaments and microtubules because the exchange rate is much slower o Not found in all eukaryotes only animals IF filaments o Nonpolar structure o Presence of conserved helical domain o Coiled coil motif o flanking the rod domain nonhelical N and C terminal domains of different sizes characteristic of each IF class o primary building block dimmer held together through the rod domains that associate as a coiled coil then make tetramers where 2 dimers are in opposite orientations o tetramers assembled end to end and interlock in long protofilaments o 4 protofilaments protofibril o 4 protofibrils associate side by side 10 nm filament o 1 filament 16 protofilaments in it o tetramer symmetric no polarity o filament based on its structure rather than its mechanism of assembly Nuclear lamins nucleus nuclear structure and organization Lecture 33 Nuclear structure and function Nuclear lamins o Network of lamin intermediate filaments meshwork extending over inner surface of nuclear envelope o Cytoplasmic filaments extend from cytoplasmic side of NPC into cytosol Nuclear lamins building blocks of nuclear architecture o Numerous pores perforate the nuclear envelope in all eukaryotic cells o Each nuclear pore formed from elaborate structure nuclear pore complex NPC large protein assembly o Made up of multiple copies of 30 different proteins nucleoporins o Lamins structural support and chromosome organization Protein synthesis and targeting nucleus o All proteins found in nucleus histones transcription factors and DNA RNA polymerases synthesized in cytoplasm and imported through nuclear pore complexes nuclear pore structure o ions small metabolites and globular proteins up to 20 40 KDa can passively diffuse through the central aqueous region of the NPC o large proteins and ribonucleoprotein complexes cannot diffuse in and out of nucleus actively transported through NPC with assistance of soluble transporter proteins that bind macromolecules and interact o contain nuclear localization signal NLS and contain fully folded with nuceloporins conformation NLS direct nuclear proteins to the nucleus o Directs proteins to cells nucleus o Cytoplasmic proteins can be localized to the nucleus when fused to a Ran GTPase drives directional transport through nuclear pore complexes nuclear localization signal o Ran Importin and conformations Monomeric G protein exists as either GTP or GDP bound o 2 importins can form heterodimeric nuclear import receptor o subunit binds to basic NLS in a cargo protein to be o subunit interacts with proteins in nuclear pore to shuttle transported into nucleus cargo