The Standard Abbreviation (ISO4) of PNAS is “Proc. Natl. Acad. Sci. U.S.A.”.
ISO 4 (Information and documentation – Rules for the abbreviation of title words and titles of publications) is an international standard, defining a uniform system for the abbreviation of serial publication titles. One major use of ISO 4 is to abbreviate the names of scientific journals. The International Organization for Standardization (ISO) has appointed the ISSN International Centre as the registration authority for ISO 4. It maintains the List of Title Word Abbreviations (LTWA) containing standard abbreviations for words commonly found in serial titles. LTWA includes more than 56,000 words and their abbreviations in 65 languages.
In conclusion, under ISO 4 standards, the PNAS should be cited as Proc. Natl. Acad. Sci. U.S.A. for abstracting, indexing and referencing purposes.