BMB 251 1st Edition Lecture 34 Outline of Last Lecture I. AxonsII. Patch-clamp recordingIII. SynapsesIV. Pre/Postsynaptic cellV. Synaptic clefVI. –Gated ion channelsVII. Parts of the cella. Organelleb. ER’sc. GolgiOutline of Current Lecture VIII. ChloroplastsIX. MitochondriaX. Types of Protein Translocationa. Gatedb. Transmembranec. VesicularXI. Nuclear Pore ComplexesXII. Ran GTPaseCurrent Lecture- Eukaryotic cell has much smaller ratio of surface area to volume  plasma membrane has too small an area to perform all necessary functions  internal membranes fix that - Chloroplast: includes thylakoid, which harbors all of molecule’s photosynthesis machinery - Compartments of eukaryotic cells can originate from invagination and pinching off of specialized intracellular membrane structures from plasma membrane. This creates organelles with interior topologically equal to exterior of the cello ER, Golgi, endosomes and lysosomes- Mitochondria and plastids evolved from small bacteria engulfed by large bacteriao Double membrane, DNA, similar sizes and structures, ribosomes- Eukaryotic cells have four distinct families: o Nucleus and cytosol- communicate via nuclear pore complexeso All organelles acting as secretory and endocytic pathways – ER, Golgi, lysosomes, endosomes, etc.o MitochondriaThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Plastids (plants only)- Protein’s amino acid sequence determines sorting signals: direct delivery to locations outside of cytosolo Most do not have a strong signal and permanently stay inside of cytosol o Many have signals transporting them between nucleus, ER, mitochondria, plastids, etc. - Gated transport: proteins move between cytosol and nucleus via nuclear pore complexes (NCP) in the nuclear envelope  selective gates with active and passive transport depending on the size of the molecules passing through - Transmembrane transport: transmembrane protein translocators directly transport specific proteins across a membrane from cytosol  topologically distinct place- Vesicular transport: membrane-enclosed transport intermediates ferry proteins from one compartment to another; they bud and pinch off from one membrane and then fuse with another membrane - Each mode of protein transport usually is guided by sorting signals (SS), which bind to sorting receptorso SS reside in stretch of amino acids, ofen on N-terminuso Specialized signal peptidases remove SS from finished protein o SS can also be internal stretches of amino acids, which remain part of proteino Each SS specifies specific destination in the cello **Even if amino acid sequence of proteins destined for same destination vary greatly, their SS are functionally interchangeable - Info required to construct an organelle doesn’t reside exclusively in DNA that specifies the organelle’s proteins- info in the form of at least one distinct protein that preexists in organelle membrane is also required and this info is passed form parent cell to progeny cell via organelle division - Nuclear envelope: encloses nucleus and DNA; defines nuclear compartmento Two concentric membranes penetrated by pore complexes - Inner nuclear membrane contains specific proteins that act as anchoring sites for chromatin and nuclear lamina (provides structural support for envelope)- Outer nuclear membrane: continuous with membrane of ER; studded with ribosomes engaged inprotein syntheses  protein transported between two membranes- Nuclear pore complexes (NPC): perforate envelopeo Let in small solutes (amino acids, nucleotides) passively and in closed conformationo Larger macromolecules take active transport to get through - Nuclear localization signals (NLS): selectively active import of proteins into nucleuso Rich in amino acids lysines and arginines (positively charged)o Can be located almost anywhere in amino acid sequence and form loops/patches on protein surface- To initiate nuclear import, most nuclear localization signals must be recognized by nuclear import receptors; they are soluble cytosolic proteins that bind to both NLS and NPC proteins o Import receptors and their bound cargo proteins bind and dissociate from one NPC protein; then rebind to the next, hopping down proteins of NPC  Once they reach the nucleus, receptor proteins dissociate form cargo and return to the cytosol- Nuclear export need nuclear export signals (NES) on macromolecules to be exported, and nuclear export rectors, which bind to both NES and NCP proteins to guide cargo to cytosol- RAN GTPase imposes directionality on transport through NPC and is found in nucleus and cytosol, so it is needed for both import/exporto Exists in two conformational states, where either GDP or GTP is boundo Cytosolic GTPase-activating protein (GAP) triggers GTP hydrolysis and converts Ran-GTP to Ran-GDPo Nuclear guanine-exchange (GEF) exchanges GDP for GTP, converting Ran-GDP back into Ran-GTPo Since Ran-GAP is located in cytosol and Ran-GEF is located in nucleus, and so the cytosol contains mainly Ran-GDP and the nucleus mainly Ran-GTPo Nuclear import receptors “hop” along NPC proteins until they reach the nucleus, the binding of Ran-GTP causes the release of nuclear protein and Ran-GTP/nuclear import receptor travel back out to cytosol, where Ran-Gap converts Ran-GTP to Ran-GTP- Some proteins have localization signals for import AND export, and their active rates of import/export determine the steady-state localization of such shuttling proteins o If a protein diffuses faster into cytosol than nucleus, it is mainly located in cytosol, and vice