Items 73-79
For each numbered description of functional role, choose the lettered cell adhesion protein that BEST corresponds. Each lettered answers may be used once, more than once, or not at all.
(A) N-CAM
(B) N-cadherin
(C) Integrin
(D) TAG -1
(E) Fasciculin II
(F) Myelin-associated glycoprotein (MAG)
(G) Proteolipid protein (PLP)
(H) PO
(I) Laminin
(J) Fibronectin
73. This cruciform molecule is required for neurite outgrowth. It is also a prominent structural protein in many basement membranes.
74. This cell adhesion molecule is stabilized by extracellular Ca 2+ but becomes subject to rapid proteolysis when extracellular Ca 2+ is removed.
75. This transmembrane integral membrane protein is a heterodimer of ?- and ?. in the ? 5 ? 1 configuration, it serves as the fibronectin receptor.
76. Intimately associated with actin via vinculin, talin, and ? – actinin.
77. This Ca 2+ -independent cell adhesion molecule is part of the immunoglobulin superfamily.
78. This protein is present on the extracellular face of mylen. It is responsible for the intreperiod line width in mylen of the central nervous system.
79. This mylen protein is highly glcosylated and is responsible for homophilic binding intraperiod line width in mylen of the central nervous system.
ANSWERS AND TUTORIAL ON ITEMS 73-79
The answers are: 73-I; 74-B; 75-C; 77-A; 78-G; 79-H. Neural cell adhesion molecule ( N-CAM ) ( A ) is the most abundant cell adhesion molecule in the nervous system. It is a member of the immunoglobulin superfamily of proteins. N-CAM is used for Ca 2+ - independent, nonspecific cell-cell adhesion mediated by homophilic interactions, i.e., N-CAMs on adjacent cells interact with one another. TAG-I ( D ), fasciculin II ( E ), mylen-associated glycoprotein ( MAG ) ( F ), and LI are also part of the immunoglobulin superfamily. These N-CAM – related molecules are less widely distributed in the nervous system than N-CAM itself. Their primary role is in directing migration of neurons or glial cells in the developing nervous system by Ca 2+ -independent, homophilic interactions.
N-cadherin ( B ) is the most important Ca 2+ -dependent cell adhesion molecule of the nervous system. N-cadherins on the surfaces of neurons and glial cells mediate cell-cell adhesion by homophilic interactions. Ca 2+ is thought to induce conformational changes in N-cadherin, thus rendering them less susceptible to proteolytic turnover.
Integrin ( C ) is another important glycosylated intergral membrane protein involved in cell adhesion by a heterophilic mechanism, i.e., interaction between two different constituents, one being the integrin ( receptor ) and the other being the ligand (fibronectin, laminin, collagen, fibrin, etc. ). These are many different clases of intergrins based on different combinations of several varieties of ?- and ?- subunits to form a heterodimer. The ligand specificity of intergrins is determined by the specific nature of the heterodimer. For example, the ? 5 ?1 hererodimer binds strongly to fibronectin ( J ) in the extracellular matrix. In contrast, the ? 6 ?1 heterodimer binds strongly to laminin ( I ), a cruciform glycoprotein of basement membranes.
It is currently thought that neutire migration along laminin-rich basement membranes in the developing nervous system is mediated by integrins in neurons. Integrins are connected to actin-rich microfilaments of the cytoskeleton by vinculin, talin, and ?- actinin.
Myelin is formed by multiple layers of Schwann cell membranes in the peripheral nervous system ( PNS ) and of oligodendroglia in the central nervous system ( CNS ). High resolution transmission electron micrographs of myelin reveal major dense lines ( formed by close apposition of the cytoplasmic faces of myelinating cell membranes ) and intraperiod lines ( formed by apposition of the extracellular faces of myelinating cell membranes ). In the PNS, intreperiod lines are 15 nm apart; whereas, in the CNS, intraeriod lines are 14 nm apart. This slight difference is due to differences in cell adhesion proteins. Forensic Comparison Microscopes Inverted Tissue Culture Microscopes Metallurgical Microscopes
In the PNS, a transmembrane glycoprotein member of the immunoglobulin superfamily, called P o resulting in a slight decrease in the thickness of the intreperiod line.
