Well, they're held together by Covalent and non Covalent interactions. So we have Vanderwal's forces that are basically the London dispersion forces between the non polar side chains of the amino acids found on these two opposing subunits. We also have ionic bonds which are basically the bonds between the negatively charged side chains and the positively charged side chains. We also have hydrogen bonds and we have a type of Covalent bond known as a disulfide bond or a disulfide bridge. This is a covalent bond that is formed between two adjacent 15 amino acids. Now, the more disulfide bonds we have inside the alpha carotene, the stronger and the more rigid that molecule is that protein is.