I am currently submitting a paper for Nature Structural & Molecular Biology. I worked on this project for the last four years and was the only one working on this project. I had everything going very well. In order to get the crystallization, I gave the plasmid construct to another laboratory which has expertise in crystallization. Fortunately, they got the crystals.
After the crystallization got successful, one researcher (say, A) from their lab came to my lab and my professor told me to teach him all the methods. I happily agreed to do so. After A went back to his lab, I got the draft of the paper from my professor, where, my name appeared as first co-author! I almost fainted. There were 20 figures in the paper. 13 were contributed by me, 4 by A and rest by 2 other people. The paper was mostly about my work. But in the contribution section, my prof and the prof of A mentioned that I and A had equal contribution. In fact it is mentioned that A was solely contributor of crystals and he equally contributed in all other results.
I discussed this issue with my prof, but he tells that he is getting pressure from A's prof to make A the first author. And my prof does not want to estrange relationship with A's prof. I am heart-broken and I don't know what to do. Please suggest me what to do.
Edit 2021: It has been quite some time since I posted this question. I accepted the authorship as recommended by my advisor. Now I feel mature and realize that maintaining a cordial relationship with one's advisor and colleagues is more important than anything else. If one is capable enough, s/he will get more opportunities to prove their worth.