One way to do an yeast transformation is by using lithium acetate, a single-stranded carrier DNA, and PEG (1). I was wondering why is the polyethylene glycol important for the efficient transformation. How does it affect the take-up of the foreign DNA? Yamakawa et al (2) showed that PEG is essential for the recovery of the cells but I couldn't access that paper to read more about it.
I found this link - it says 'we don't really know'. It says PEG binds DNA, I assume shielding the membrane from its negative charge and allowing internalization to happen.
I would guess that the amphipathic nature of PEG, being partly hydrophobic, also helps soften up the membrane. Interestingly, if you increase the PEG concentration beyond the limits, it decreases the efficiency of the procedure.
I found this article that might answer your question:
Shigeyuki Kawai1, Tuan Anh Phan1, Emi Kono1, Kazuo Harada2, Chihiro Okai2, Eiichiro Fukusaki2, and Kousaku Murata1 (2009). Transcriptional and metabolic response in yeast Saccharomyces cerevisiae cells during polyethylene glycol-dependent transformation. Journal of Basic Microbiology 2009, 49, 73 – 81
I think this is the correct link: http://onlinelibrary.wiley.com/store/10.1002/jobm.200800123/asset/73_ftp.pdf?v=1&t=hfux1ljn&s=2c2619c78848056e5a51eacebf4489f34657fec6
I hope it helps!