STRUCTURAL-ANALYSIS OF THE MURINE CELL-ADHESION MOLECULE L1 BY ELECTRON-MICROSCOPY AND COMPUTER-ASSISTED MODELING

In the present study we have analysed the morphology of two fragments with apparent molecular weights of 180 and 140 kDa (L1-180 and L1-140) derived from the extracellular region of the murine neural cell adhes ion molecule L1. The fragment L1-180 consists of almost the entire ext racellular part of the molecule, and is built up of six immunoglobulin -like and five fibronectin type III-like domains. Fragment L1-140 lack s one-half of the third, the fourth and the fifth fibronectin type III -like domains. By electron microscopic analysis of rotary-shadowed mol ecules, L1-140 and L1-180 revealed fibrillar structures 31-43 nm long and 7-12 nm wide with one pronounced globular terminal domain. As dete rmined by complex formation with an L1 antibody, this terminal part of the molecule is formed by the fibronectin type III-like domains. The individual structures showed variation and complexity, and four distin ct aspects were identified. These different forms probably represent t wo-dimensional projections of the same three-dimensional helical struc ture. Computer-assisted modelling of the L1 molecule, i.e. the protein backbone,showed no strong intramolecular interaction between the diff erent fibronectin type III- or Ig-like domains, suggesting that the fo rmation of the globular part of the molecule is probably achieved by p rotein-carbohydrate and/or carbohydrate-carbohydrate rather than prote in-protein interactions. In addition, our model proposes that interact ions occur within the interfaces between the different domains. The hi ghly conserved amino acid residues in these regions point to the neces sity of maintaining the orientation between the different domains.