Final answer:
Microtubule-related tubulin is abundant in neural tissue, actin in muscle tissue, and keratin in skin, hair, and nails. Keratin would be the least soluble protein to extract. Soluble proteins feature hydrophilic amino acids on their surface and hydrophobic amino acids inside, while membrane proteins have a mixture to interact with the lipid bilayer.
Step-by-step explanation:
A vertebrate tissue that would be an excellent source of tubulin would be neural tissue because neurons rely heavily on microtubules for axon guidance and transport. Actin can be abundantly found in muscle tissue, as it plays a crucial role in muscle contraction forming microfilaments. Keratin, on the other hand, would be abundantly present in tissues such as the skin, hair, and nails since it is a structural protein that contributes to the protection and integrity of epithelial cells.
As for solubility, keratin would likely be the least soluble and most difficult to extract due to its fibrous and insoluble nature, which provides structural strength to cells. When isolating tubulin, one might find proteins associated with microtubules as contaminants, such as MAPs (microtubule-associated proteins). In a preparation of actin, possible contaminants might include proteins that regulate actin polymerization like profilin and gelsolin.
Looking at the placement of amino acids, one would expect soluble proteins to have hydrophilic amino acids on their surfaces, which interact with the aqueous environment. Inside the protein, one would find hydrophobic amino acids that are shielded from water. In proteins embedded in lipid bilayers, you would expect a mixture of hydrophobic residues in contact with the lipid tails and polar or charged residues in contact with the lipid head groups or aqueous domains.