Various classes of microtubule-associated proteins (MAPs) are expressed in eukaryotic cells. Whereas some MAPs bind specifically to the microtubule plus ends or the minus ends (centrosomes), many MAPs bind along the microtubule lattice. The latter category includes both enzymatically active MAPs, such as microtubule motors or the microtubule-severing protein katanin, and structural MAPs such as the MAP2/tau or stable tubule only (STOP) protein families. This article focuses on the classical, microtubule lattice-binding structural MAPs in the MAP1 family, which are best known for their microtubule-stabilizing activity. Most knowledge on MAP1-family proteins has been derived from studies in rodents and, unless noted otherwise, insights from rodents are expected to apply for mammalian family members in general. The MAP2/tau family of classical MAPs is encoded by distinct, apparently unrelated genes and has been reviewed in an earlier issue of Genome Biology 1.

Most vertebrate genomes (including human, mouse and rat) contain three family members, MAP1A, MAP1B and MAP1S, which are encoded by separate genes (see Table 1 for chromosomal locations of the human genes) 234. The shortest MAP1 protein, MAP1S, is also known as VCY2IP1 or C19ORF5. A search for proteins with sequence similarity to MAP1A or MAP1B proteins (summarized in Figure 1a) shows that there are three apparent MAP1 family members in bony fish (L.D. and S.H., unpublished observations), but the functional significance of these isoforms is unclear. Fish MAP1-family proteins have been reported to be only about 25% of the size of their mammalian counterparts 5. No obvious ortholog of any MAP1-family protein is present in Caenorhabditis elegans or more primitive organisms, but a single protein related to MAP1A and MAP1B, called Futsch, can be found in Drosophila melanogaster 6. Futsch differs from vertebrate MAP1A and MAP1B isoforms in that it contains a repeated central domain with homology to vertebrate neurofilaments 6. As classic neurofilaments are absent from the Drosophila genome, it may be that Futsch is an ancestral precursor of neurofilament proteins.

Vertebrate MAP1-family genes span multiple exons. Alternative splicing has been reported only in mammalian MAP1B genes 3; its functional relevance is unclear. Sequence similarity between distinct MAP1-family proteins in an individual organism is most prominent in the extreme amino and carboxyl termini (approximately 85% similarity at the amino-acid level). Sequences with significant similarities are also found in Drosophila Futsch (approximately 60% similarity to rat MAP1A or MAP1B), but it is not clear from this information whether the Drosophila protein is an ortholog of either MAP1A or MAP1B (Figure 1a).

An accessory protein chain that can be found in MAP1A and MAP1B protein complexes is derived from the LC3 gene encoding MAP1 light chain 3 7. LC3 and related proteins do not show significant sequence similarity to MAP1A and MAP1B and are not usually considered to be part of the MAP1 protein family. At least seven distinct LC3-related genes are found in humans (Table 1, Figure 1b), and various orthologs of these genes are found in both highly developed and simpler eukaryotes. LC3-related genes are related to the yeast ubiquitin-like gene AUT7 (ATG8) 8 and are thought to play a role in autophagy. One LC3-related gene has been predicted in archaea (hypothetical protein ST0261 in Sulfolobus tokodaii). No orthologs of the MAP1 and LC3 families are found in prokaryotes.

The MAP1A, MAP1B and MAP1S polypeptides are each translated as larger proteins that are then processed by proteolytic cleavage near the carboxyl terminus, leading to the generation of heavy chains (MAP1A-HC of 350 kDa, MAP1B-HC of 300 kDa and MAP1S-HC of 100 kDa) and light chains (LC2 of 28 kDa from MAP1A, LC1 of 32 kDa from MAP1B and MAP1S-LC of 26 kDa) 4910. The light chains generated by MAP1A (LC2) and MAP1B (LC1) can interact with both MAP1A and MAP1B heavy chains 11. For MAP1B, light-chain binding has been mapped to a 120 kDa fragment within the amino terminus of the heavy chain 10. LC3 can also interact with the MAP1A and MAP1B heavy chains 7. The exact stoichiometric composition of MAP1A and MAP1B heavy and light chains has not been determined, but in the case of MAP1B, a ratio of MAP1B-HC:LC1:LC3 of 1:2:0.2 has been estimated 12.

All four light chains can bind microtubules by themselves 4713, and the MAP1A and MAP1B heavy chains both contain additional sequences that bind microtubules (Figure 2) 14151617. Amino acids within these microtubule-binding domains are diverse, including both positively and negatively charged residues. Interestingly, the MAP1B heavy chain was found to suppress the microtubule-binding activity of its light chain 18. For MAP1A, the contributions of the heavy and light chains for microtubule binding are less clear. In kangaroo PtK2 cells, exogenously expressed MAP1A light chain (LC2) was sufficient by itself to bind and stabilize microtubules 13. In contrast, in green monkey COS7 cells, both MAP1A light and heavy chains were required 19.

In addition to microtubule-binding activity, the MAP1A, MAP1B and MAP1S light chains can also bind filamentous actin (F-actin) 41318. The microtubule- and F-actin-binding sites on the MAP1A and MAP1B light chains map to different sequence regions. Microtubule binding is confined to the amino terminus of the light chains, and a direct F-actin interaction has been localized to the carboxyl terminus. Furthermore, an exogenous carboxy-terminal fragment of MAP1A and MAP1B light chains colocalized with F-actin in stress fibers of non-neuronal cells 1318. It is yet not known if a single MAP1 unit can bind both cytoskeletons at the same time and thus crosslink the two cytoskeletons.

Structural details about MAP1-family proteins are largely unknown. Both microtubule and F-actin binding have been mapped to regions of about 120 amino acids in the MAP1B light chain, but no further structural details or critical amino-acid residues related to these interactions have been identified. The only structural data available are derived from electron microscopy of preparations treated by the rotary shadowing technique. These studies suggest that MAP1A is a flexible, elongated protein 20, whereas MAP1B appears to be a rod-shaped, elongated molecule with a terminal, round globular domain 21. No information about secondary structures in either molecule is available. Moreover, predictions suggest that mammalian MAP1A and MAP1B and Drosophila Futsch are natively unfolded (L.D. and S.H., unpublished observations; predictions were calculated using FoldIndex 22). Although over 50% of their entire protein sequences are predicted to be unstructured, some folded regions might exist in the extreme amino termini of the heavy chains.

MAP1 family members and their splice variants have specific regional and temporal expression patterns in the nervous system. MAP1B is highly expressed during early neuronal development and gradually diminishes during maturation 23. In developing cultured neurons, MAP1B protein is localized to axons, as well as their precursors (so-called 'minor neurites') 24. Furthermore, MAP1B is especially enriched in growing axons 2324. MAP1A is predominantly expressed in adult neurons, where it localizes preferentially to dendrites 11. MAP1S is expressed in various tissues including mouse brain 4.

Two decades after their original discovery, many functions of MAP1A and MAP1B have been uncovered in vitro and in vivo. Knockout animals and functional assays suggest specific roles of MAP1 family members in both the development and the degeneration of the nervous system. Structural details of MAP1-family proteins are largely unknown, however, and apparent functional redundancies and crosstalk with other MAPs and cytoskeletal regulators make it difficult to pinpoint the exact function(s) of individual MAPs in vivo. Furthermore, the variety of upstream regulatory pathways and downstream effectors provide a major challenge to fully understanding MAP1A and MAP1B function. Fortunately, certain key pathways controlling MAP1A and MAP1B activity have been identified, although little is yet known about MAP1S. A broader and more precise analysis of phosphorylation and other posttranslational modifications still needs to be carried out, however, in order to fully understand MAP1A and MAP1B function in signaling networks controlling neuromorphogenesis.