Background: Goats (Capra hircus) have always been a source of fiber for human use and hold an important place in international high-end textiles. Fiber diameter is the most concerning economic indicator for producers. Understanding the formation mechanism of fiber diameter and its related key proteins can help optimize and control the production of cashmere. Methods: Cashmere goats (n = 36) of the Alpas (n = 18) and Alxa (n = 18) breeds, with a similar age (2 years old) and live weight (25-26 kg), were selected from the Yiwei White Cashmere Goat Breeding Farm, Erdos, Inner Mongolia. Using phenotypic indicators, we evaluated the diameter of the cashmere fibers in Alxa and Alpas goats. We also used electron microscopy to examine the cashmere fiber's structure and label-free liquid chromatography-tandem mass spectrometry to determine the protein content of the two cashmere fibers. The proteins affecting fiber diameter were identified and analyzed by Western blot, Co-Immunoprecipitation, and bioinformatics analysis. Results: The average diameter of the Alxa breed was smaller (p < 0.05) than that of the Alpas breed (Alxa's cashmere vs. Alpas' cashmere). Proteomics technology enabled the highly confident detection of 171 proteins. A total of 68 differentially expressed proteins were identified in the two types of cashmere; 131 proteins were specifically expressed in Alpas goats, and 40 proteins were specifically expressed in Alxa goats. A key protein group that could cause variations in fiber diameter was found using the protein-protein interaction network. To ascertain the reason for the variation in fiber diameter, a structural study of the major protein groups was carried out. Conclusions: KRT10, KRT14, KRT17, and KRT82 are the main proteins impacting the diameter difference, and they have a substantial effect on the average fiber diameter.
